Ah ha! Found it.
The Player's tracking still "remembers" that Zombie hanging out in the upper-right corner. Thus, he considers himself in combat... which means "keep party together" mechanics are not allowed to engage.
For this to happen, you would need to encounter and retreat from an enemy, without encountering and killing any other enemy before trying to use quick travel of some kind again (characters only remember one enemy at a time, so if you met and killed something else, you would end up with no memory of enemies and be able to quick travel properly.)
I think the fix I will try for this is to limit the distance a memory is valid. I don't want to clear memory immediately upon leaving LoS: the whole point is so that characters can pursue out of sight enemies.
(If this seems like it'd be irrelevant for the player, I should point out that demon AI is group based with leaders making most group decisions like where to travel to, thus your summoned allies rely on the tracking attached to the player to determine their movement.) But, I probably should set a way for a memory to be given up on (particularly for the player, since unlike actual AIs, the player may not necessarily be even trying to investigate a remembered enemy.)
Not sure exactly what shape the fix will take yet, but I'm working on it.
Thanks again for the save file geminimax, and wiz and everyone else for the many reports.
We're finally closing in on a fix now, I think.