Heya! Sorry for the very long delay in my response, been really busy of late. I'm finally starting to get back into the groove of modding and wanted to take a look at this.

Very interesting thing you found, and unfortunately, this looks to be an issue with the game itself and how Animation Masks are used. When you begin to turn, the animation mask is removed and re-applied; this seems to break the current animation state and causes your character to start the turn animation instead of just rotating their upper body. Almost all animation "fixes" being applied here are animation masks, as the alternative would be to replace all animation files which is a lot to say the least. Player movement, aiming, and the current animation state is controlled in the Java core, I cannot fix this via a mod in Lua.

With Fancy Handwork disabled, you can easily reproduce this same issue by aiming a pistol while holding a bag in your left hand.

I could potentially fix this for when you are just aiming a pistol in the main hand by not re-applying the mask, however, holding any item in your left hand would still cause this issue currently.

Doesn't affect my gameplay negatively though. Visual bugs without any gameplay break isn't really that bad. Hopefully this can be fixed in the future when the game engine improves though.