The bug:

Spawning mobs*** against a wall will cause them to get stuck or suffocate.

*See 'affected mobs' for list.

Affected mobs:

• Spider
• Polar Bear
• Turtle
• Donkey
• Ghast
• Mule
• Skeleton Horse
• Zombie Horse

Steps to reproduce:

1) Make a wall (can be any size)

2) Grab a spawn egg of one of the mobs listed in 'affected mobs'.

3) Use the spawn egg on the wall.

Conclusion:

Hitboxes of entitys are too big causing them to glitch into the block.

See the screenshots I attached for more information.

Video: https://streamable.com/vrzah (video file was too big)