I have now tested the shopkeeper in 1.15 .. actually they almost seem to work ... only they do not move.

do you already know if it will be difficult to fix the problem?

I do not know if it helps, but if I select a / shopkeeper npc, then it also moves in 1.15.1

Update:Unfortunately, I have to say that there is also a problem. They work well but noticed that after a while they disappeared. However, they are still available in the save file. You cannot put any new ones where they were. Very funny

runs without problems so far. I'm happy that everything is running normally again. A great job of yours

Fixed with v2.9.0

Regarding the citizens npc issues: I wasn't able to reproduce any disappearing yet. But could you try to place some regular citizens npcs nearby and check if these disappear as well when this issue occurs? In that case this might be a Citizens issues.