Found in Version:
In TR_m3_TG_Fentus5, the quest where you have to rescue Antio Florane of the Thieves' Guild from a Dwemer ruin, Antio Florane did not spawn -- the staff that he was supposed to be stealing does spawn.
After experiencing the bug, I found the exact same story here: https://www.reddit.com/r/Morrowind/comments/bewtt9/help_with_old_ebonhearts_thieves_guild_questline/?st=jwmmxmhl&sh=a28966eb
Attempts to force the quest to progress through the Journal console command break the quest immediately afterwards (where TG members disappear).
Another oddity that might be related: the room immediately past where the staff can be found contains a Dwarven Sphere -- on reloading, the Sphere relocated to be behind the remaining collapsible rocks rather than in front of them.