The bug
Stray cats can spawn inside non full blocks (like stairs, slabs, chests, etc) and some can suffocate them (like dirt path).

To reproduce:

1. Create a Superflat world with no villages (Generate Structures: off) and in peaceful mode
2. Stay still on the ground and use these commands:

   /fill ~50 ~-1 ~50 ~-50 ~-1 ~-50 minecraft:oak_slab[type=top]
   /fill ~2 ~-1 ~2 ~-2 ~-2 ~3 air
   /setblock ~ ~-1 ~ minecraft:pink_carpet
   /tick freeze

3. Fill the trench with 5 beds.
4. Spawn 5 villagers in the trench.
5. Place a repeating command block with this command and set it to always active

   /execute at @e[type=minecraft:cat] run setblock ~ ~1 ~ minecraft:diamond_block

6. Go inside the hole with the pink carpet, unfreeze the game and wait until it appears in chat that a block was changed.

   /tick unfreeze

   If the message appears, look for a diamond block around you and see if under it there is a cat trapped inside a slab.

What I expected to happen was...:
A cat spawning above the slab or not spawning at all.

What actually happened was...:
A cat spawned inside the slab.