For some reason, the one-handed attack animations in First Person is broken with the left arm glitching at the end of onehandattackleft and onehandattackleft_chop. I'm using the Melee Combat Additions mod which adds third person attack animations, I removed them temporally to see if they caused the bugged animation and no, they didn't. I have been looking for new animations for 1st Person attacks to replace the broken vanilla one.