The bug
When switching into spectator mode, the skin changes to an older skin of mine that I haven't used since last Christmas. This does not happen in 1.14.4.
Interestingly, the skin doesn't actually seem to be downloaded and saved in the skin assets folder.
I confirmed that this happens regardless of whether skins have been downloaded before.
Related issues
testing discovered
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Possibly caused by MC-164076? But I wouldn't know how my skin would look like my Christmas skin through that bug. Also, the problem persists after I changed to my Halloween skin.
I think what's happening is that it's showing fully transparent pixels as fully opaque. When you erased the santa hat(?), the color data remained and is now being drawn fully opaque (then made transparent due to spectator mode)
[media]For example my head shows what seems to be black around the head, because I haven't drawn and erased anything there
That's definitely possible, and the most likely cause, actually. I think I might have improved my regular skin (that I had before) by using the xmas skin as a template, but I don't remember anymore. Unfortunately, I don't have the xcf file anymore though, and from the png I cannot check the hue of transparent pixels, or at least GIMP says that they're all white.
This is no longer happening, probably just caused by MC-164076 and my skin being messed up.