Hi Team

So this issue has been around for Magma cubes since, probably 1.16 launch actually. However, it's still happening and now I've caught overworld Slimes doing it too. Reporting together as I assume they share 90% of their code.

Also based upon the Wiki's there's a good chance this is actually a parity issue.
"A slime needs roughly a 3×2.1x3 free space to spawn. The area in which the slime spawns must be clear of solid or liquid obstructions. When a slime attempts to spawn, the game checks for the space requirement of a large slime, and the size is determined later. Therefore, since large slimes are slightly taller than 2 blocks, no slimes, regardless of size, are able to spawn at all in 2-block-high areas." Minecraft Fandom - Slime I can tell you I'm getting plenty of small and medium slimes in this clearing... Magma Cubes page lists the same spawn requirements. But same results in game, lots of small and mediums in 2 high cuttings with regular intrusions by larges embedded in the floor.

1. Large Slime's Spawning in 2 high space. So, I've had a few of these spawn in this space, this time I saw it happen. So the Slime is spawning above the solid blocks with the top of it's head in the solid block above which kills lighting on the entity entirety causing it to flash black for a second. Then it does this awkward half hop and sinks into the floor...

Aside from spawning in the first place. That's the other thing that should be impossible, mobs should not be able to just sink into solid blocks, IMO.

2. Large Magma Cubes spawned "embedded" in the floor because the tunnel is only technically two high. Pretty sure I've seen them appear already in the solid blocks although it's possible the same sequence happens as with Slimes described above.