even if they didn't want to load user fonts, it should fall back to a renderable font when rendering the NSAttributedString in a NSTextView - not to the "LastResort" font. For me this looks very much like a bug - especially as it has worked fine for many years.
BTT already tries to workaround this by requesting a non HTML format like rtfd instead, but some apps only put a HTML version into the clipboard.
I'll add a check to see whether the last resort font is used and if so, use the standard system font instead.