My skin is showing up as classic when I have clearly chosen slim.

I logged in onto my Minecraft world and my skin is showing up with two black pixels. This happened today and was really frustrating. When I changed my skin it remained classic. I looked it up and the only reason I could find was a) I had chosen the classic option (which I hadn't because I didn't have this problem the previous days.) and b) I was in a version where slim skins didn't exist which was wrong, I was on version 1.15.2. I apologize if you think that this is not a worthy complaint but my OCD cannot stand the two-pixel black gap in the arms. I care a lot about how I look in-game.