In the Galadon rewrite and the Manor rewrite at least, all the mobs do not drop their offhand (shield, polearm, weapons, etc) when they receive the 'crunch' effect from a kotegaeshi.

I know this is a terribly limited sample, but without anything else to go against for the moment, I have this for you:

New Battlement guard from Galadon:

You grab hold of a battlement guard's wrist and twist it violently!
With a sickening crunch, you apply kotegaeshi to a battlement guard's wrist!
With her broken wrist, a battlement guard can no longer hold onto a lightweight heater shield.
Your kotegaeshi devastates a battlement guard!
A battlement guard has a few scratches.


New gate guard mob from Targeth's Manor:

You grab hold of a gate guard's wrist and twist it violently!
With a sickening crunch, you apply kotegaeshi to a gate guard's wrist!
With his broken wrist, a gate guard can no longer hold onto a whale-bone spear.
Your kotegaeshi wounds a gate guard.
A gate guard has a few scratches.

The Prison Captain in Galadon with the polearm (when it isn't illusionary)

The dwarf guy with the Warg Shield and the Flail named Dragon Heart in the Manor

1) that's not a polearm :P

2) the mob does not always load with the shield so that could be the confusion

You grab hold of Draekar, the grizzled veteran's wrist and twist it violently!
With a sickening crunch, you apply kotegaeshi to Draekar, the grizzled veteran's wrist!
With his broken wrist, Draekar, the grizzled veteran can no longer hold onto a huge shield crafted from a warg skull.
Your kotegaeshi wounds Draekar, the grizzled veteran.
Draekar, the grizzled veteran has a few scratches.

Edit to add: The above instance occured while the mob was wielding the weapon and wearing his shield. If it is occurring for you in an instance where he is wielding the flail and wearing the shield, then go ahead and show the log of it.

868hp 700m 971mv 21600tnl (-30.91%) 12 AM civilized k 2.
You step out of the shadows.
You trample around loudly again.
Your slash MUTILATES Draekar, the grizzled veteran!
Draekar, the grizzled veteran has a few scratches.

868hp 700m 971mv 21600tnl (-30.91%) 12 AM civilized
You misdirect Draekar, the grizzled veteran's pummel with a fake to your left.
Draekar, the grizzled veteran has a few scratches.

868hp 700m 971mv 21600tnl (-30.91%) 12 AM civilized kot

You dodge Draekar, the grizzled veteran's beating.
Your slash EVISCERATES Draekar, the grizzled veteran!
Draekar, the grizzled veteran parries your punch.
Draekar, the grizzled veteran has a few scratches.

868hp 700m 971mv 21600tnl (-30.91%) 12 AM civilized You grab hold of Draekar, the grizzled veteran's wrist and twist it violently!
With a sickening crunch, you apply kotegaeshi to Draekar, the grizzled veteran's wrist!
Your kotegaeshi EVISCERATES Draekar, the grizzled veteran!
Draekar, the grizzled veteran has a few scratches.

868hp 701m 973mv 21600tnl (-30.91%) 12 AM civilized
You dodge Draekar, the grizzled veteran's beating.
Draekar, the grizzled veteran parries your slash.
Your punch EVISCERATES Draekar, the grizzled veteran!
Your slash MUTILATES Draekar, the grizzled veteran!
Draekar, the grizzled veteran dodges your slash.
Draekar, the grizzled veteran has a few scratches.

868hp 701m 973mv 21600tnl (-30.91%) 12 AM civilized l draek

You parry Draekar, the grizzled veteran's beating.
You dodge Draekar, the grizzled veteran's beating.
Your slash EVISCERATES Draekar, the grizzled veteran!
Your punch EVISCERATES Draekar, the grizzled veteran!
Your punch MASSACRES Draekar, the grizzled veteran!
Draekar, the grizzled veteran deflects your slash with his shield.
Draekar, the grizzled veteran has some small but disgusting cuts.

868hp 701m 973mv 21600tnl (-30.91%) 12 AM civilized Towering above most others in the room, this giant has seen his share of
combat. His coal-black skin is covered with fine, reddish-orange hair
except where scars from long-healed wounds are visible. His nose is a
mangled mess, having been broken repeatedly with one nostril that has been
split open. His left hand ends in a scarred heap with the last two fingers
having been cut off at their base. The tousled hair atop his head is no
more pleasant, tangled and knotted as it falls in random directions. He
glares angrily at the guards lounging about and bellows loudly at them,
accentuating his statements with the heft of his weapon.
Draekar, the grizzled veteran has some small but disgusting cuts.

Draekar, the grizzled veteran is using:
<worn as shield> a huge shield crafted from a warg skull
Draekar, the grizzled veteran has some small but disgusting cuts.

So this is one of those fun situations where mobs are different than PC's. NPC's do not have a defined "primary" and "secondary" hand for items. Since the mob only loaded with the shield, and not a weapon, the mud is counting the shield as its "primary" slot.

To further test this, I attempted to wield a weapon with the same mob while it had the crunch kot effect and was wearing the shield to start with. It returned the message that you cannot use a second item while your wrist hurts so much.

Hopefully that helps.