Skin defaults to Classic 4px arm when game is relaunched when having a file as a skin

Whenever I load a 3px Alex-style slim skin, during the current session it works. But when I relaunch the game, it becomes 4px, causing the black bars on the arms to appear, which is a bit distracting.

 

https://drive.google.com/file/d/1rA_IBeKdUNnxDbItcR9SRG-ie-zancbe/view?usp=sharing