When changing my skin, I decided to use the smaller 64x32 variant, that used to be standard, to save on file size because I did not need the second layer or different textures for each arm or leg.

When I tried in-game, there were black bars on the left arm in the full player model much like when using a 3px arm skin for the 4px arm model.

This does not affect the first person arm.

Screenshots and the skin in question attached.