I noticed while trying to make a fox powered chicken cooker that this bug does not often happen for me under normal circumstances, however if there is a carpet on top of a slab that the chicken is growing up into, it moves into the wall and suffocates. the chicken is safely not suffocated if it merely grows up on a slab in an enclosed space without a carpet.
even with one chicken, it still gets shoved into the wall
EDIT: it appears that this is the case not only for carpets, but for mostly any block being above the slab the animal grows up on. it makes them move into the wall
I'm having something similar, with villagers stealing eachothers work stations. I had 2 composters yet when 2 babies grew up they chose those composters.
I'm on version 1.15.2 of java edition, on realms, no mods
Can confirm in 1.18.1
Notably, it seems more specifically that slow falling and jump boost do not take effect if you are in control of the ridden entity. (i.e if you take the saddle off of a horse or stop using a carrot on a stick, slow falling works perfectly fine)