I've tried adding the filter where you suggested and then added an "AND" after it (it errors out otherwise) - But not only does it not limit the 1 option, it doesn't show ALL of the products though (it shows 70 of them but not ALL the options are showing, but some are -- There should be 78 products including all the options - for whatever reason, it doesn't show 8 of them). It's very strange. When I add the filter at the END of the statement, and I have an "AND" BEFORE the filter - it shows different results (40 instead of 70 or 78) - but it shows (for some products) more options than the other way of doing it. It's definitely not working right.
Is there anything else I can try? Can I open a support ticket? Or is it the same support I'd get here?