How to remove "Additional Criteria" in Report Editor?

Hi,

I'm working on a report which is a copy of existing report (not sure if it's a default report or created by someone else). 

There is an Additional Criteria (see image) on two Data Sources which I cannot remove/delete effectively, meaning it reappears when I save, close, and open the report again. 

I didn't find any relation to Additional Criteria anywhere in the report (except for Parameter, which can be successfully removed). 

Does anyone know the reason behind this issue? I'm obviously missing something but I've run out of ideas.

Any help would be much appreciated.