I made a few custom NPCs once just for fun, and let them walk around the castle. All was working fine, but one time I noticed that one of my custom NPCs had mysteriously disappeared. Same happened to another one soon after.
I couldn’t figure out what the problem was, until, while I was talking to my creatures in the Menagerie, I saw a custom NPC just walking constantly to the left, through walls and all obstacles and everything. Intrigued, I closed the dialogue I was having with my creature and as soon as I did the NPC stopped moving and returned to normal behavior.
I decided to make another custom NPC to experiment with this issue, and found out that what happens is that, when a dialogue in the Menagerie is opened while a custom NPC is walking, that NPC will continue to walk in that direction constantly, moving through everything, until you close the dialogue. I don’t know if this happens with ALL dialogue or just with the Menagerie dialogue but so far I’ve only confirmed the latter.