BUG - Very strange behaviour when grouping page names from a GA4 data source

[Ktrs]

Hi all,

I'm trying to understand a very strange behaviour in Looker Studio, reporting on data coming from a GA4 connector.

Here is my issue. I'm counting my sessions and views per day and per Page Cluster. Page Cluster is a calculated field I added into the data source. It's a simple regex-based if-else logic:

...where page_content Event is a custom parameter in GA4 that contains the page name (/path).

With no filters set, here is how the numbers look like:

Now I want to exclude all pages that contains the phrase "/kontakt/".

First, I use a filter based on the page_content Event parameter. I simple exclude all pages that contain /kontakt/ from the filter selection:

That decreases the numbers, naturally.

So far so good.

Now I reset the first filter, and I use the second filter instead. The second filter is based on the Page Cluster calculated field that I described above.

Now watch this! As soon as I EXCLUDE that cluster, look how the sessions per day change: 

How on earth is it possible that I exclude a group of pages and the numbers increase three-fold? The totals are still reasonable but the sessions per day are going through the roof. As soon as I use that calculated field, the numbers don't make any sense anymore, while totals are still correct. 

Any explanation will be greatly appreciated since this has been driving me crazy for a while.

Thanks, Ercan