Filter by stack.function doesn't seem to filter?

I’m trying to filter a list of issues, that are caused by the same c# exception, by a function name found in the stack trace, using the stack.function: filter.

It seems that the filter recognizes the function names that occur, meaning if I put in a function that is not found, there is no result. However, given a function that does occur on some of the stack traces, the filter simply returns all of the issues, as if no filter was used.