Some mobs don't have appropriate "stopping ranges" when attempting to attack an enemy.

stopping range = when a mob stops because it thinks it can attack

attacking range = when a mob actually attacks an enemy.

I thought this bug was isolated to baby zombies but it seems to apply to everything. Its just most noticeable for baby zombies because they are much smaller than the normal zombie size. They are using the stopping range of a normal zombie and this is making them miss. They stop to attack when they are too far away to even attack. This does however apply to normal zombies and other mobs just far less noticeable. I have provided clips of this bug. I recently tested this because I thought MC-147516 was a fix to it. But its still in the game.

https://streamable.com/omr8d9

https://streamable.com/fj1dqj

The stopping range of mobs should be adjusted to be consistent with their attacking range. You don't even have to be moving for this bug to occur with baby zombies. But you can get it to repetitively occur if moving.