Not sure if that's a case of dual-wielding or just a lack of a message that a monster unwields something in between.

Also, it's nowhere said that greater balors would have only two hands. ;)

Game did not say he punched you... Even with the hands being busy, he likely has wings, horns, hooves, etc...

The creator asked me in Facebook to fill a bug report. I did.

I'd assume this applies to all monsters, as monster inventories in ADOM are not particularly specific. IIRC, there's other weird things that can happen like a monster can equip four suits of armor at once.

I don't think that preventing monsters from attacking while dual-wielding shields is the best solution, as this is the kind of thing that could be easily to exploit. Better just to prevent them from equipping two shields, or have them only equip the highest DV/PV one and remove the other.

As far as I understand, then monster equips something he gets DV/PV, autocurse on item trigegrs but that's just it. No slots occupied, no intrinsincs, no damage boost from weapon , etc.

It would be nice if this mess was fixed, but dual shields is not really main thing I would worry about (mobs dying from burning hands with red dragon scale mail equipped breaks immersion worse).

Monsters will no longer dual-wield shields as the monster AI is not really up to such advanced tactics. Thus I decided to forbid this completely.