fyi:  always to null checks first (why, I don't know, it has been an issue with CR for years).   I hope this helps.

I am just saying if you have a isnull filter then it should be first before any other filter.   Also check in your options (worded a little different in some versions) for 'Convert Other NULL Values to default'.  There is also the same check in Report Options (which I believe overrides the Option settings).  Then there is the 'Exception for NULLS' in the formula editor.

Nulls always seems to an issue with me, so I almost always set an formula while developing a report that will show me the result of a null check (i.e.  In a formula just put isnull({wkrofile.closed_date} and see what you get).

I hope this helps.