Using a spawn egg on zombies, zombie villagers, piglins, or zombified piglins in a filled boat causes the baby to suffocate in the ground

For example, if you spawn a baby zombie by using a spawn egg on a zombie in a boat that is filled (i.e. there's another entity in the boat), the baby zombie will go into the ground and suffocate.

Video

Minecraft 20w51a - Singleplayer 2021-01-05 17-40-33.mp4

Code analysis

Code analysis by [Mod] Avoma can be found in this comment.