The bug
Entities riding other entities that don't need air will dismount each other when in more than two blocks of water. Entities can still ride skeleton horses underwater. but not other undead mobs.
So undeads can't ride any other undead mobs underwater except skeleton horses.
Since undeads and things like armor stands don't need air it doesn't make sense for them to dismount when under water.
How to reproduce
Find some deep water (5 blocks deep), and the run:
/summon minecraft:zombie_horse ~ ~ ~ {Passengers:[{id:zombie}]}→ ❌ Notice how the zombie dismounts the zombie horse when submerged underwater
Linked issues
Attachments
Comments 14
Can confirm in 20w51a. Here are some steps to reproduce this issue.
Steps to Reproduce:
Find some deep water (5 blocks deep), and the run command:
/summon minecraft:zombie_horse ~ ~ ~ {Passengers:[{id:zombie}]}→ ❌ Notice how the zombie dismounts the zombie horse when submerged underwater.
As of 1.19.4-pre2 this only affects zombie horses, and that case is is invalid/WAI as per MC-134484 (although I strongly disagree with that resolution), so this issue can be marked as fixed. Due to the fix for MC-260461, this mechanic is now controlled by the #dismounts_underwater tag (which should arguably be called #evicts_passengers_underwater IMO), which includes zombie horses.
Please do not mark unreleased versions as affected.
You don't have access to them yet.