Filter expression behaving differently in report design and in the published report

I have two different shared datasets for the standard SEP Alerts cube, both with the same filters. The only difference between the datasets are the fields returned. When I import the datasets into a report without further filtering they work as intended. However, if I only want to have the data for the previous month, I use the filter expression =MonthName(Month(DateAdd("m",-1,Now()))). This works as intended when I preview the report, but when I publish the report one of the datasets is empty.

The expression =MonthName(Month(DateAdd("m",-1,Now()))) = 'July' returns true when previewing, but returns false when the report is published. If I filter the datasets for just July instead of an expression it works as intended. The bug occurs for all numbers, not just -1.