IsNull Function

I'm having wrong results using IsNull function in the selection formula of a report that I was requested to enhance, however, I've created a brand new report with the same criteria as a clone of the existing report, and the Isnull function is pulling the right results in the new report unlike in the existing one

I'm pretty sure that the two report are the same criteria, tables, fields, and everything, I've tried to look up any potential hidden conditions but got nothing

Any hint would be appreciated