When a mob that has a passenger mob riding it becomes submerged underwater, the rider will be dismounted and the two will be separated
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I had tested this with Creeper riding Zombie, Creeper riding Iron Golem, Armour Stand riding Zombie, Ravager riding Phantom and Skeleton riding Spider, the last was the only one that didn't dismount, probably because the spider floated on top of the water
violine1101
Thank you for your report!
We're tracking this issue in MC-72440, so this ticket is being resolved and linked as a duplicate.
That ticket has already been resolved as working as intended, which means this is not considered a bug and won't be fixed. Please do not leave a comment on the linked ticket.
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That's definitely WAI with horses, other mobs I'm not sure