I too have been using PaintDotNet for years, probably about 15 years.
I donated along the way and purchased from MS Store. While I know I have not paid what I would have to pay to the other guy, it is still irritating to spend further money. I am 78 years young and limited income makes me grumpy sometimes. Sorry . . .
Just checked MS Store, searched for paintdotnet, paint, paint.net, the MS Store says no record found. I really am confused.