For whatever reason (I don’t know how any of this works) deleting the first sans-serif doesn’t solve the issue, but I do agree with you that it makes sense as the final fallback.
In my case, as you can see, my sans serifed font is not the wacky Star Trek one I’m getting.
If I uncheck “Allow pages to choose…”, then I get my preferred fonts instead of this (as yet) unidentified font.