Weird recent animation bug causing animation idle loops to shift around?

I’ve been animating weapons by making actions and then holding those actions using idle loops.
However recently, my animations have been acting up and have been looking like this.

One of my friends who reanimated the tools had their character look like this.

In the latter gyazo, when the character brings his weapon to the front, the weapon isn’t supposed to shift up and down.

Additionally, in the former gyazo, when my character initially puts his weapon to the left shoulder, it’s not supposed to shift back and forth either.

This is a very recent bug that seems to have only occurred a day or so ago. Does anyone know why this is happening?