Did not wish to post while alive, so here it goes:

Necro mobs seems to always "not assist" in the second round of combat, when the player starts the combat. So, how to reproduce:

1- Make undead - zombies, greater undead, flesh golem;
2- Attack something;
3- Watch them assist the first round;
4- Watch them not assist the second round;
5- Watch them assist normally from third round on.
6- Rinse and repeat.

Logs showing it:

PvE situation:
civilized <716hp 971m 679mv 23060tnl (-25.60%) 3 PM> c vamp orc
Your deathly touch *** DEMOLISHES *** an orcish castle raider!
An orcish castle raider has a few scratches.

civilized <843hp 931m 679mv 23060tnl (-25.60%) 3 PM> (FIRST ROUND, ASSIST)
An orcish castle raider's defilement DISMEMBERS you!
You parry an orcish castle raider's defilement.
Your searing cut MUTILATES an orcish castle raider!
A flesh golem screams and attacks an orcish castle raider!
A flesh golem's smash maims an orcish castle raider!
A ghoul screams and attacks an orcish castle raider!
A zombie of Ralmoth screams and attacks an orcish castle raider!
A zombie of Ralmoth's punch MUTILATES an orcish castle raider!
A zombie of Ralmoth's punch maims an orcish castle raider!
A zombie of the old dwarf screams and attacks an orcish castle raider!
A zombie of the old dwarf's punch misses an orcish castle raider.
A zombie of the dwarven smith screams and attacks an orcish castle raider!
A zombie of the dwarven smith's punch MUTILATES an orcish castle raider!
A zombie of the old dwarf screams and attacks an orcish castle raider!
A zombie of the old dwarf's punch EVISCERATES an orcish castle raider!
An orcish castle raider is covered with bleeding wounds.

civilized <787hp 931m 679mv 23060tnl (-25.60%) 3 PM>
An orcish castle raider tries to bash you but stumbles through you as you become completely insubstantial briefly.
An orcish castle raider is covered with bleeding wounds.

civilized <787hp 931m 679mv 23060tnl (-25.60%) 3 PM>

Autosaving.
An orcish castle raider is covered with bleeding wounds.

civilized <771hp 940m 730mv 23060tnl (-25.60%) 4 PM> (SECOND ROUND WITH MOBS NOT ASSISTING)
An orcish castle raider's defilement misses you.
An orcish castle raider's defilement maims you!
Your searing cut maims an orcish castle raider!
An orcish castle raider is covered with bleeding wounds.

civilized <738hp 940m 730mv 23060tnl (-25.60%) 4 PM> c vamp
Your deathly touch *** DEVASTATES *** an orcish castle raider!
An orcish castle raider is covered with bleeding wounds.

civilized <878hp 900m 730mv 23060tnl (-25.60%) 4 PM> (THIRD ROUND - NORMAL AGAIN)
A flesh golem's smash maims an orcish castle raider!
A flesh golem's smash MUTILATES an orcish castle raider!
A zombie of Ralmoth's punch EVISCERATES an orcish castle raider!
A zombie of the old dwarf's punch MUTILATES an orcish castle raider!
A zombie of the dwarven smith's punch misses an orcish castle raider.
A zombie of the dwarven smith's punch misses an orcish castle raider.
A zombie of the old dwarf's punch MUTILATES an orcish castle raider!
You parry an orcish castle raider's defilement.
Your searing cut MUTILATES an orcish castle raider!
Your searing cut MUTILATES an orcish castle raider!
An orcish castle raider is gushing blood.

----------------------------------

PvE situation:
civilized <771hp 1024m 704mv 23060tnl (-25.60%) 9 PM> c vamp sorria
Your deathly touch *** DEMOLISHES *** Sorrialana!
Sorrialana has some small but disgusting cuts.

civilized <898hp 984m 704mv 23060tnl (-25.60%) 9 PM> (FIRST ROUND, ASSIST)
Sorrialana fades into existence.
Sorrialana's electrical discharge maims you!
Your searing cut decimates Sorrialana!
Your searing cut devastates Sorrialana!
A zombie of a gorgeous silver-haired elf screams and attacks Sorrialana!
A zombie of a gorgeous silver-haired elf's punch EVISCERATES Sorrialana!
A zombie of a gorgeous silver-haired elf's punch DISMEMBERS Sorrialana!
A zombie of the old dwarf screams and attacks Sorrialana!
A zombie of the old dwarf's punch misses Sorrialana.
A zombie of the old dwarf's punch DISMEMBERS Sorrialana!
A flesh golem screams and attacks Sorrialana!
A flesh golem's smash EVISCERATES Sorrialana!
A flesh golem's smash misses Sorrialana.
A zombie of the dwarven smith screams and attacks Sorrialana!
A zombie of the dwarven smith's punch DISMEMBERS Sorrialana!
A zombie of the dwarven smith's punch DISMEMBERS Sorrialana!
A ghoul screams and attacks Sorrialana!
A ghoul's claw DISMEMBERS Sorrialana!
A ghoul's claw DISMEMBERS Sorrialana!
A zombie of the old dwarf screams and attacks Sorrialana!
A zombie of the old dwarf's punch DISMEMBERS Sorrialana!
Sorrialana is writhing in agony.

civilized <864hp 984m 704mv 23060tnl (-25.60%) 9 PM> (SECOND ROUND WITH MOBS NOT ASSISTING)
Sorrialana utters the words, 'yazfuz yucandus'.
A zombie of a gorgeous silver-haired elf is surrounded by a pink outline.
You parry Sorrialana's electrical discharge.
Your searing cut MUTILATES Sorrialana!
Your searing cut devastates Sorrialana!
Sorrialana is convulsing on the ground.

civilized <864hp 984m 704mv 23060tnl (-25.60%) 9 PM> (THIRD ROUND - NORMAL AGAIN)
A zombie of the dwarven smith's punch MASSACRES Sorrialana!
A ghoul's claw EVISCERATES Sorrialana!
Sorrialana is DEAD!!
You receive 0 experience points (100% bonus).
Sorrialana's feet are sliced from her dead body.
Supple boots from the pelt of an aged cooshee's illusion is broken and fades away.
A carved ivory scepter's illusion is broken and fades away.

--------------------------------------

PvP situation...
civilized <811hp 1067m 792mv 23060tnl (-25.60%) 11 PM> call 'sigil of pain' rhi
The deep bellow of a rhinoceros echoes through the air.
You call upon the Codex and carve a Sigil of Pain onto a black rhinoceros.
Your sigil of pain maims a black rhinoceros!
A black rhinoceros is covered with bleeding wounds.

civilized <811hp 1007m 792mv 23060tnl (-25.60%) 11 PM> (FIRST ROUND, ASSIST)
Your searing cut wounds a black rhinoceros.
Your searing cut decimates a black rhinoceros!
A sullen gloomwraith screams and attacks a black rhinoceros!
A sullen gloomwraith's claw mauls a black rhinoceros.
A flesh golem screams and attacks a black rhinoceros!
A flesh golem's smash devastates a black rhinoceros!
A flesh golem's smash maims a black rhinoceros!
A zombie of the old dwarf screams and attacks a black rhinoceros!
A zombie of the old dwarf's punch MUTILATES a black rhinoceros!
A zombie of the old dwarf's punch MUTILATES a black rhinoceros!
A zombie of the old dwarf screams and attacks a black rhinoceros!
A zombie of the old dwarf's punch maims a black rhinoceros!
A zombie of the old dwarf's punch maims a black rhinoceros!
A zombie of the dwarven smith screams and attacks a black rhinoceros!
A zombie of the dwarven smith's punch devastates a black rhinoceros!
A zombie of a cloaked figure screams and attacks a black rhinoceros!
A black rhinoceros's crush *** DEMOLISHES *** an Imperial Centurion!
A black rhinoceros's crush *** DEMOLISHES *** an Imperial Centurion!
A black rhinoceros's crush *** DEMOLISHES *** an Imperial Centurion!
A black rhinoceros is gushing blood.

civilized <811hp 1007m 792mv 23060tnl (-25.60%) 11 PM> (SECOND ROUND WITH MOBS NOT ASSISTING)
An Imperial Centurion's punch MUTILATES a black rhinoceros!
Your searing cut mauls a black rhinoceros.
Your searing cut wounds a black rhinoceros.
A black rhinoceros's crush *** DEMOLISHES *** an Imperial Centurion!
A black rhinoceros is writhing in agony.

civilized <811hp 1007m 792mv 23060tnl (-25.60%) 11 PM> c vamp
Your deathly touch MUTILATES a black rhinoceros!
A black rhinoceros is writhing in agony.

civilized <840hp 967m 792mv 23060tnl (-25.60%) 11 PM>
Lohjun has arrived.
A black rhinoceros is writhing in agony.

civilized <840hp 967m 792mv 23060tnl (-25.60%) 11 PM> (THIRD ROUND - NORMAL AGAIN)
The edges of a black rhinoceros's being stretch and twist and she returns to normal.
An Imperial Centurion's punch MASSACRES Aelithara!
A zombie of a cloaked figure's punch DISMEMBERS Aelithara!
Aelithara is mortally wounded, and will die soon, if not aided.
A zombie of a cloaked figure's punch MANGLES Aelithara!
Aelithara is DEAD!!
You land the killing blow against Aelithara.
Aelithara's head is shattered, and her brains splash all over you.

Sure costed me a few deaths/missed kills during my time.

Thanks for the attention, hope it helps.

That is correct.