The bug
When a "Size:0" slime and magma cube rides on some mobs the rendering position is too low.
Steps to reproduce
Use the following command:
/summon minecraft:blaze ~ ~ ~ {Passengers:[{id:slime}]}
All affected mobs:
Blaze
Cave spider
Chicken
Cow
Creeper
Donkey
Drowned
Elder guardian
Enderman
Evoker
Ghast
Giant
Guardian
Hoglin
Horse
Husk
Illusioner
Iron golem
Llama
Magma cube(Size:1+)
Mooshroom
Mule
Panda
Pig
Piglin
Piglin brute
Pillager
Polar bear
Sheep
Shulker
Skeleton
Skeleton horse
Slime(Size:1+)
Sniffer
Snow golem
Spider
Stray
Strider
Trader llama
Turtle
Villager
Vindicator
Wandering trader
Warden
Witch
Wither
Wither skeleton
Wolf
Zoglin
Zombie
Zombie horse
Zombie villager
Zombified piglin
I think this has something to do with the size of the hitboxes and how the slimes and magma cubes ride other mobs.