But the tag "Foursome - M/M/M/M" very much contains the letter "F". It's why that first F is highlighted in the shown tags.

I think you're correct though. My guess is that it prioritized these searches because it has an exact match of singular Fs in them, like what they've typed. All male four/fivesome both haves Fs, and would probably be included if it includes more results, but they're scored lower since there's no lone Fs.