The Crimson Creep

In 1.18.1

Confirmed

They also sometimes run back and forth when attacking the player.

In 20w16a

Confirmed. This seems to happen to villagers you've levelled up.

Can confirm. Also sometimes exiting a boat turns it invisible

Confirmed for 1.14, and it plays when standing still at my base. 

They also constantly run in and out of houses

This is intended because they share AI

This is what I'm currently dealing with in my world while trying to test a custom boss battle.