As reported by @99of9 :
it seems not to correctly handle |property_threshold=1, instead it returns 0% for anything with under 20 instances)
Here's a demonstration of the threshold issue though: https://www.wikidata.org/w/index.php... . When I require the group sizes to be over 2, even the rows about those which are size 3 change. If they're Property item count is under 3, that cell also gets set to 0.