Incorrect player skin while offline

When I play Java connected to the internet on my Mac, my character has the skin I chose in the launcher (Steve), but when I’m offline, it forces the Sunny skin onto my character without changing anything in the launcher. I have tried to select, reselect, and change the skin multiple times in the Minecraft Launcher but to no avail.

I doubt this is something which only affects my laptop, but I haven’t had the chance to test that so have added details anyway in case they might be helpful.