So after beating the game in vanilla, I decided to do another run with the modpack, and I've encountered this problem where any slave pulled from the brothel by pixies to the slave cage, milking station, or delivery room, doesn't seem to know how to path back there.
Edit: So after playing around further, I think I understand the issue. Rather than moving slaves to any available slave cage, the pixies seems to try to place the slaves exactly in the same spot they were before. As I was moving slaves around to different cages and to the monsters, I noticed that then and only then did the slaves idling on the milk machines and delivery room then get moved to their original slave cage spots. Interesting