This particular image has not been hosted on lemm.ee before, as I’m the one who uploaded it to pixelfed.social

I'm guessing that means you're also the image creator, otherwise you wouldn't be sure nobody had uploaded it here before, right?

In any case, my first thought would be to try to do a test, replicating the event to see if the same thing happens again. Unfortunately, it looks like pixelfed.social is closed to new accts, so I suppose it would need to be someone already with an acct there. Feel like giving that a whack?

If so, there's a junk community here that would make for a good testing ground. I just whipped up a test image with the same dimensions as yours, and right about the same size here.

if it's not convenient for you to do a test at this time, maybe you could clue me in as to how to make a pixelfed.social acct?