The filters section says A namespace or a prefix term is not a filter because a namespace will not run standalone, and a prefix will not negate. This seems empirically untrue. On EnWP I get the following number of results for each of these queries, as expected:
incategory:"LGBT-related musical films"
: 58incategory:"LGBT-related musical films" prefix:"Hello"
: 2incategory:"LGBT-related musical films" -prefix:"Hello"
: 56
So it seems like negating a prefix does work. Am I misunderstanding what this is trying to say? For now I added a {{dubious}} tag.