EDIT: Trying it again today, hoping it will work out before tonight when my old instance goes down. So far, it very much looks like yesterday is repeating itself. My personal guess is, the mod promotion doesn't properly reach lemmy.world for some reason, and then later, the other two instances sync with lemmy.world as the authority (after all the community is on there) and say (rightfully) "hey, the instance the comm is on doesn't have those accounts as mods, we better remove the privileges again!".