The bug
When you open the horse's inventory while holding any item/block in your main hand, the held item will seem to bob in the background.
How to reproduce
Linked issues
relates to
Comments

I did test it on survival mode and it also happens.
Fixed in 20w06a, probably fixed with MC-93198.

Relates to MC-273770. I believe this was caused by inventory changes behind the scenes.
Probably caused by creative mode.