Thank you for the information. I've done some testing locally and see a similar behavior (filters for "not null" being added in to the drilldown report when I summarize a column in the top level report), but it's different enough that I want to get some additional information from you and try some additional testing.
Are the columns "CoverKey" and "CoverObjectKey" summarized in the report you are drilling down from? What about in the report you are drilling down to?
Do you have any filter conditions set on the report you are drilling down from that use these columns? In the report you are drilling down to?
Is there a way to remove the filters that are applied from the parent report to drill down report. Eg if we drill down from a report by default we see the conditions on the drill down report. Is there a way to hide them.