I think I've stumbled upon an AI glitch. If this has already been posted, please forgive me, fully reading this thread is an inhuman task.
There's evidence to suggest that the AI becomes paralyzed when it is at war with another AI, and this AI falls apart due to complete dominion loss, resulting in undefended neutral provinces.
Among other circumstances, I've once observed Pythium warring against Mictlan, and Mictlan falling apart. (Mictlan often falling apart is another glitch you probably know about.)
Pythium did absolutely nothing for an unspecified number of turns. It even had an army with its pretender surrounded by now-neutral, undefended provinces, yet the army didn't take one of them. I was able to take every single neutral province there was, imprisoning the pretender and his army within my provinces. When Pythium finally awoke of its "coma", it was no match at all, probably due to its long paralyzation.
I believe you can awaken a paralyzed nation by attacking it, and that a paralyzed nation will awaken by itself after considerable time.
My theory: The AI doesn't recognize that the war is at an end and looks out for provinces of the deceased AI to conquer. It ignores the neutral provinces since its only priority is taking enemy provinces.
A closely related problem (or the same problem?) is: AI nations will never conquer neutral undefended provinces. (At least those resulting from nation collapse.)
Please excuse my superficial description of events, but I have not enough of an idea what happened "under the hood".