Nah it was solved with a Defender update.
I didn't try to download any files. I thought you meant you couldn't even access the site. In any case, the resolution may have been the result of some user or paint.net providing feedback to MS, or Defender itself reporting it. Or, since the security community shares malware data (not their proprietary algorithms) with each other, MS might have learned through a different solution provider. Either way, I am glad it got resolved.
Again, not common, but false positives do happen.
Oh sure. No solution is immune to them and for sure, they are annoying. But they are MUCH BETTER than false negatives!
Curiously, in looking at
this AV Comparison, Avast, AVG, McAfee, Norton, Panda, F-Secure, Malwarebytes and Trend Micro (some of the big names) got some of the poorest scores for FPs. Panda had 25, Malwarebytes had 32, and Trend Micro had 52

compared to the average, which was 11.
Defender was tied with Kaspersky in 3rd place with just 2. VIPRE only 1, and Total Defense had a perfect 0 this go around.
Clearly, that is another example where paying (typically over and over again

) for expensive security protection gets you no better protection than with some of the free solutions. Oh well, that's for a different discussion.
Years ago, before Malwarebytes Premium went to a subscription fee price structure, I took advantage of their very last, lifetime license offer and bought several licenses at huge discounts ($10 each, if I remember right). And for sure, I encounter FPs with it rather frequently.

Worse, IMO, is when they tag my "wanted" and totally safe programs as PUPs (potentially
unwanted programs). That really irritates me.
I still like Malwarebytes and I do recommend the free version for supplemental, double-checking scans. But I would not pay for or recommend the Premium version today and that is in large part, due to the many FPs.
But, that too is for a different discussion.