villagers dont interact with potion stand when it's on a slab

i tried getting the villager to become a cleric with a potion stand but no matter how close i put them to it they just tried interacting with other items untill i put the potion stand of the floor

to do glitch
1. put down a slab on the top half
2. put potion stand on that slab
3. the villager now cannot interact with potion stand