Hello and welcome @davidpiano.
I just ran into the same problem, trying to search for the odd-sounding phrase “shortened suite” while excluding “suit”, and finding that when I use -suit it has no effect; either way I get the same set of results, most of them containing the word I specifically excluded and not even containing the word I searched for.
The issue which is cropping up here is one of stemming; the minus operator itself does work, for example see:
https://www.mojeek.com/search?q=cats
vs
https://www.mojeek.com/search?q=cats+-dogs
But in this case suite is being stemmed down to suit, so the -word is having no effect on the search itself. We have a few stemming issues that we’ve taken down recently and they’ve been added into the queue to fix.