Quote:It seems to be connected in some way to the instant 'teleport' they do whenever you get too far away and some internal timer expires, as was the case in 1.09d.
I agree that it most likely has something to do with the teleport, but there must be some other factor(s) as well. I've been playing alot lately, and my mercs teleport often to catch up (playing in normal/NM, I run ahead of them all the time). This bug very rarely occurs though. I still have no real idea what actually triggers it.
EDIT: Occasionally the merc just vanishes completely, and never reappears in the same area. The portrait is still there, but the merc is not on screen(or map), and doesn't reappear after a few seconds. Only returning when a wp(or teleporter) is used. This one happens even less often than the monsters ignoring merc though.