I believe if the time is short enough between fleeing / returning to a battle, it is irrelevant if you are invisible.

LOG ------------------------------

An elven mercenary's slash misses a priestess.
A priestess's punch scratches you.
A priestess's punch hits you.
Your slash grazes a priestess.
A priestess is covered with bleeding wounds.

civilized graveyard <31%hp 89%m 100%mv 5677tnl (-6.71%) 8 AM>
flee
On a Path
Tall sets of evergreen trees loom over you as you traverse this north-south
path among the graves. To the far north, you can make out a small structure,
while to the south lies the center of this necropolis. A small tombstone is
set at the foot of a large statue of the Lady Nordewin.


(Translucent) Something is here, though only faint wisps of greenish light can be seen.
You flee from combat!

civilized graveyard <31%hp 89%m 99%mv 5677tnl (-6.71%) 8 AM>
You aren't fighting anyone.

civilized graveyard <31%hp 89%m 99%mv 5677tnl (-6.71%) 8 AM>
cast invis
You fade out of existence.
You have become better at invis (77%)!

civilized graveyard <31%hp 85%m 99%mv 5675tnl (-6.67%) 8 AM>
s
The Junction of Paths
You are standing at the junction of the main paths through the graveyard.
To the northeast and the southwest, you can see two large statues looking
across it. A large fountain gushes here, and you can see that several
people have passed through this place. Paths lead north, south, east, and
west. To the north you can see a small structure, and to the south lies the
gate to Galadon.


A steaming pile of a mourner's entrails is lying here.
A large fountain sits in the courtyard.
A woman clothed in long, white robes slowly moves about the area, chanting softly.
A young woman stands near a grave, quietly sobbing.

civilized graveyard <31%hp 85%m 98%mv 5675tnl (-6.67%) 8 AM>
A priestess yells 'There is no rest for the wicked, ANON, and that includes you!'
A priestess's punch grazes you.
A priestess is gushing blood.

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I believe it's a bug because I went back after dying from this (invisible), and the mob did not attack me even as I looted my corpse and re-equipped.

I know some mobs can see through invisibility, and I don't believe the mob in question does.