The bug
Baby mules will not spawn when right-clicking on an adult mule with a mule spawn egg.
Caused by the fix of MC-60634
Linked issues
relates to
Comments
The ticket title was changed. When I tested it, it only said that mules are affected. And I was able to confirm that.
Confirmed and changed the title back. Mules are the only affected animals.

Zombies are also affected by this, but that's a (needlessly) controversial one.
Zombies are not breedable animals.

Zombie horses, skeleton horses and polar bears are also not breedable animals.
Yeah, zombies eggs should be able to spawn baby zombies but that just my opinion. Anyways I can reproduce this issue.
Can't reproduce