Made a test skin in a skin editor. Painted right leg only, slim model (Alex).

Loaded the skin in minecraft (custom skin), notice that left leg is also painted on slim model! Fat model (Steve) is rendering correctly.

This happens with any slim model skins. Putting the skin into a skin pack does not help either. The left leg no matter what copies the right leg.

I attached the test skin if you want to see for yourselves.

Bug was also present in older versions since character creator update, didn't have time to report it, but seemingly no one has noticed this yet!