So. I recently opened a ticket about the villager spinning on our vanilla realm server.

As you asked, I tryed to reproduce this bug in solo vanilla and it was successful.

I made 3 differents configurations, the middle one is exactly what we have on our server, (Oak slab on water)
the right one is exactly the same but with full Oak planks (I put 2 Oak slab to make a full block) and the last one is exactly the same as the middle one but in the air.

After hiting the bell each NPCs managed to go inside there house excepte for the middle one who spins in place.

So my guess is that Slabs placed on water cause a in bug the path finding of the NPCs (probably because of the water's physics)

I Hope it helps !

Picture of what I tried in annexe