Hi,
Thank you very much for the details, it helps a lot!
I have found the issue. The problem is, that on the search sources, there are a few more post types selected as in the filter itself. For example the Campaigns is selected as the source, but it’s not in the filter options. This results in Campaings being always included in the results – regardless of the filter state.
Select the same post types in the filter as in the Search Sources will resolve the issue.