Crimulus

Additionally, MC-211217 is marked as Resolved, so I’m not sure how that ticket could be considered as actively tracking the issue.

I’m not sure if it is a duplicate or not. At a minimum, the information from this ticket needs to be copied over, because the drowned jockey spawning was working prior to 26.1 and then it broke again, meaning MC-211217 was resolved (either intentionally and undocumented or accidentally) and then regressed again in 26.1 (or maybe late 1.20 versions – I’m not sure when it changed).

Slow motion demo of the baby drowned attempting to mount a baby chicken under a trap door instead of a top slab.

I’ll run a test, but I’m confident it’s not the hitbox. The baby zombies, zombie villagers, husks, and zombified piglins all have the same hitbox and do not dismount when their head is in a slab. Additionally, the same thing happens with baby chickens which are 0.3 blocks shorter, effectively achieving the same test as switching to a trap door which gains 0.25 blocks of headroom.

Please let me know if there is more information I can provide.

Here is video of a baby drowned attempting to mount a chicken and failing. The block above the chicken is a top slab (oxidized cut copper, if it’s relevant) and is not waterlogged. You can see the drowned spawn on the left and pick up the egg from an adjacent chamber on its way teleporting to the chicken but never actually riding it.

Here is a test world. I’ll work on a video if I can figure it out, but I’m not sure it will show much more than the screenshot does.