When I go to select "Choose main hand" and switch to the left hand, it suddenly becomes a slim-hand, when the Minecraft character's arm is supposed to be "Classic" sized (aka steve body type)

This dosn't make sense as the right arm remains perfectly same Classic body type sized while the left arm magically changes to an alex's arm, which shouldnt happen unless you change your minecraft character's body type. Another issue is when your left hand is supposed to be classic but changes slim, it remains "classic" when you switch to 3rd-person, perspective however if you play in 1st-person it becomes slim yet again.