Looks like all tools are messed up, the curves aren't symmetrical at all. It's happening in MS Paint too. I'm starting to believe this is caused by a Windows update.
Thanks for all the answers guys.
I hope the devs fix this issue. It's weird that it started happening to me only a few days ago, but to some people it started happening years ago.
As I already said, the circles were PERFECTLY FINE before, without anti-aliasing. This started happening a few days ago. I really need 1-pixel wide circles, not anti-aliased ones.
This is apparently a 15x15 circle.
This never happened before, the circles were perfectly symmetrical. I tried installing an older version of paint.net but it doesn't help.