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 in peaceful mode with no villages (Generate structures: disabled)
2. Stay still on the ground and use these commands:

   /tick freeze

   /fill ~50 ~-1 ~50 ~-50 ~-1 ~-50 minecraft:oak_slab[type=top]

   /fill ~2 ~-1 ~2 ~-2 ~-2 ~3 air

   /fill ~2 ~-2 ~2 ~-2 ~-2 ~2 white_bed[part=foot,facing=south]

   /fill ~2 ~-2 ~3 ~-2 ~-2 ~3 white_bed[part=head,facing=south]

   /summon villager ~ ~ ~2 {Passengers:[{id:"minecraft:villager",Passengers:[{id:"minecraft:villager",Passengers:[{id:"minecraft:villager",Passengers:[{id:"minecraft:villager"}]}]}]}]}

   /execute as @e[type=minecraft:villager] run ride @s dismount

   /setblock ~ ~-2 ~ repeating_command_block{powered:1b,Command:"/execute at @e[type=minecraft:cat] run setblock ~ ~1 ~ minecraft:diamond_block"}

   /setblock ~ ~-1 ~ minecraft:stone_pressure_plate

   /tick unfreeze

3. Wait for a message in chat about a block begin changed.
   • 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.