I suspect an i18n issue. What locale are you using, and what is the decimal separator character supposed to be in your locale?

It looks like your "Digit grouping symbol" field is empty. I'm sure that's standard in some locales, though not for US English. I don't know how to make that field empty; when I try, Windows says it's invalid. So I wonder if your locale sets that separator to some kind of Unicode character that, in a proper renderer, is equivalent to no character at all. If that's the case, then I'm guessing refterm could handle that character as easily as it handles VT escape codes. But this does lend some weight to the position that Casey was oversimplifying things a bit.

well, whatever is different in your settings, mine renders normal https://imgur.com/dGI5W2S

EDIT: I am sorry for the attitude, changed "wrong" to "different"

Their settings aren't wrong, just different, likely because of differing standards for digit grouping across locales. So this is a case that refterm clearly doesn't support. This case by itself doesn't invalidate refterm's approach to rendering, but I can see why the team at Microsoft, knowing that there are many such cases, would favor abstraction over the absolute best possible speed.

which is exactly the point.