There seems to be a bug on headwear with this resourcepack in 1.21.3 where the motion of the headwear seems to be double the motion of the head. Through testing, I was able to determine the culprit and it was quite easy to fix.
In player.jem and player_slim.jem, the following lines need to be removed to resolve this:
"headwear.rx": "head.rx",
"headwear.ry": "head.ry",
"headwear.rz": "head.rz",
"headwear.tx": "head.tx",
"headwear.ty": "head.ty",
"headwear.tz": "head.tz",

Before and after:

please add some clip

My fix was a bruteforce method and not one that resolved all problems I think. The changes requested in the below pull request by erinsc resolve the underlying problem with all parts (albeit it is a change of the no facial expressions one so you might need to account for that as well if accepting the pull request)
#212

My fix was a bruteforce method and not one that resolved all problems I think. The changes requested in the below pull request by erinsc resolve the underlying problem with all parts (albeit it is a change of the no facial expressions one so you might need to account for that as well if accepting the pull request) https://github.com/IthanMendoza/Fresh-Moves/pull/212

What they did in the pull request was replace this:

"headwear.rx": "head.rx",
"headwear.ry": "head.ry",
"headwear.rz": "head.rz",
"headwear.tx": "head.tx",
"headwear.ty": "head.ty",
"headwear.tz": "head.tz",
"right_sleeve.rx": "right_arm.rx",
"right_sleeve.ry": "right_arm.ry",
"right_sleeve.rz": "right_arm.rz",
"right_sleeve.tx": "right_arm.tx",
"right_sleeve.ty": "right_arm.ty",
"right_sleeve.tz": "right_arm.tz",
"left_sleeve.rx": "left_arm.rx",
"left_sleeve.ry": "left_arm.ry",
"left_sleeve.rz": "left_arm.rz",
"left_sleeve.tx": "left_arm.tx",
"left_sleeve.ty": "left_arm.ty",
"left_sleeve.tz": "left_arm.tz",
"jacket.rx": "body.rx",
"jacket.ry": "body.ry",
"jacket.rz": "body.rz",
"jacket.tx": "body.tx",
"jacket.ty": "body.ty",
"jacket.tz": "body.tz",
"right_pants.rx": "right_leg.rx",
"right_pants.ry": "right_leg.ry",
"right_pants.rz": "right_leg.rz",
"right_pants.tx": "right_leg.tx",
"right_pants.ty": "right_leg.ty",
"right_pants.tz": "right_leg.tz",
"left_pants.rx": "left_leg.rx",
"left_pants.ry": "left_leg.ry",
"left_pants.rz": "left_leg.rz",
"left_pants.tx": "left_leg.tx",
"left_pants.ty": "left_leg.ty",
"left_pants.tz": "left_leg.tz"

with this:

"headwear.rx": "0",
"headwear.ry": "0",
"headwear.rz": "0",
"headwear.tx": "0",
"headwear.ty": "0",
"headwear.tz": "0",
"right_sleeve.rx": "0",
"right_sleeve.ry": "0",
"right_sleeve.rz": "0",
"right_sleeve.tx": "0",
"right_sleeve.ty": "0",
"right_sleeve.tz": "0",
"left_sleeve.rx": "0",
"left_sleeve.ry": "0",
"left_sleeve.rz": "0",
"left_sleeve.tx": "0",
"left_sleeve.ty": "0",
"left_sleeve.tz": "0",
"jacket.rx": "0",
"jacket.ry": "0",
"jacket.rz": "0",
"jacket.tx": "0",
"jacket.ty": "0",
"jacket.tz": "0",
"right_pants.rx": "0",
"right_pants.ry": "0",
"right_pants.rz": "0",
"right_pants.tx": "0",
"right_pants.ty": "0",
"right_pants.tz": "0",
"left_pants.rx": "0",
"left_pants.ry": "0",
"left_pants.rz": "0",
"left_pants.tx": "0",
"left_pants.ty": "0",
"left_pants.tz": "0"

in both player.jem and player_slim.jem.
Now thinking about it, my brute force method basically did that (as I'm pretty sure 0 is assumed in the code if nothing is given.) but just for the headgear. My skin had no outerwear on the rest of it so that's probably why I didn't see the issue on the rest of the body.

This is where the change comes from:
https://github.com/Traben-0/Entity_Model_Features/blob/master/.github/1.21.2%2B%20%20changes.md

Yeah sorry i just realized i removed the facial expressions xd.
I just copy pasted my "fixed" code into my fork and thought i deleted some older code that wasn't necessary.
I also accidentally put in a trailing comma in one place which broke the slim_player file.

In summary I'm not a professional. instead of merging you really should just modify the original files, its not a difficult fix