I've been chasing the black screen battle bug for a bit and think I found the cause, shared my findings with the dev - it has to do with carrying people in court that don't have permission to be in court. When you leave the court while carrying someone, you will return to court with them. The problem is that if they don't have permission to be in court, when you return to court and they get warped without being called through interaction, the warp onto your shoulder doesn't save their position in management mode. So they get left behind in court instead of warping back. The battle camera averages the positions of the invader and defender to place itself, so their position in the court throws the camera into the void. I think the eating and bathing issue might also be related - they can't navigate to the stockpile or baths because they're trapped in court - but haven't confirmed.

The gist of it is - make sure you drop anyone you're carrying before leaving court and it shouldn't happen, hopefully. You should be able to repro it in a new game by assigning a worker, denying them from court, initiating court interaction, picking them up, leaving court, returning to court, dropping, then leaving again - they disappear from management mode and the black camera bug happens when their tile defends. If you go back to court, they're there but shouldn't be, and immediately navigate into a corner trying to push through the wall