The animation itself is completely basic.
There are no components, there is no other scripts, there is nothing occurring. This is a fresh character blank completely new nothing in any way shape or form interacting with it.
https://imgur.com/a/Q1OqjGB
Video of the "pop".
I have been trying for hours to nail down a reason for this to be occurring but I cannot find a single reason. I have tried remaking every single thing, bone structure, animations, character, reimported everything, restarted Unity, its still happening.