Hopefully removing automatically will not happend. I remember I have stuff bookmarked and start looking for there. And if I don’f find it I will be very very confused.
Keeping unworking bookmarks is useless, though. So if such bookmark will be deleted I must be informed about it. But what if there is some administration reason for moving then that topic/post may come back, but I don’t have that bookmark anymore. And moving to PM and moving to private categories are two very different cases — what if I’m part of those cases why would there be need to erase my bookmark.
So this isn’t so streamlined question at all. And I don’t see this as a bug. More or less matter or how Discouse is working.