I'm recreating this ticket because MC-266030 was 'invalid'...

Let me be more clear in my explication.

To reproduce the bug, you have to:

1. Summon a donkey and a horse with the tags {NoAI:1} and {Tame:1}. (Because you can't tame NoAI horses/donkeys).
2. GIve them both a saddle.
3. Get on the donkey.
4. Move the donkey so that the donkey's hitbox touches the horse's one.
5. Get out of the donkey.
6. Get back on it.

It seems that all the collisons are applied upon re-going on the donkey.
I tested on the UNMODIFIED minecraft Java edition version 1.20.2... The attachement is the same.

Thanks for your understanding.