Custom group data does not match the equally filtered data in Looker (also wrong compared to GA4)

Question

this is no question on a programming level but if the issue can be solved by using programming, I would appreciate any input.

We recently created a dashboard that can be used by local employees. The idea was that each country can filter by their specific website version and see the resulting data. (e.g. hostname/us/en/=United States; hostname/fr/fr/=France, etc.). So I created a custom group at the data source level. Everything looked good until last week when we realized that the data filtered by the custom group is almost double the actual count of sessions and active users for that directory...This was compared with the data in GA4 and a simple chart in Looker.

I already checked REGEXP_EXTRACT, GA4 the GTM settings and also if other data sources are messing with the chart, but in the end I compared the data filtered by custom group (dimension: Group Test 4) with the data not filtered by custom group in Looker and this discrepancy was shocking. The configuration of the underlying data filter is exactly the same. Full Page URL starts_with e.g. hostname/us/en/.

How can I fix this?

Any ideas, workaround or help in general would be greatly appreciated. Please let me know if you need more info. Thank you.

I think it is behaving similar to this issue: https://www.reddit.com/r/GoogleAnalytics/comments/17w5hsk/looker_studio_case_function_with_unexpected_counts/

Custom Group Test 4 vs. exact same filtered chart in Looker

The custom group "Group Test 4" is set up like this:

`CASE

WHEN CONTAINS_TEXT(Full page URL, "/us/en/") AND Hostname = 'www.buschvacuum.com'

THEN 'United States'

ELSE 'Other'

END`

I tried regexp_extract to get the page path like suggested here: https://www.reddit.com/r/GoogleDataStudio/comments/155rurl/trying_to_group_data_by_page_location/ but the outcome was the same.

The first table is set up in the backend exactly the same as the second. Only difference is that one is using custom group and one the direct filter applied to the chart. Both are using the same data source (with the custom group). The first table is not filtered (apart from excluding Thank you Pages) and the second table is filtered to match the custom group setup of Group Test 4= Include Full Page URL contains /us/en/ and Hostname equal to www.buschvacuum.com. (also excluding Thank you Pages)

The measurement of sessions and active user is the same. I just double checked if I have somewhere while trouble shooting changed the measurement/aggregation of Sessions or Active users but not that I can see it. I also exported the table to Google sheets to count the Sessions myself and end up the same... Maybe the users/sessions are kind of counted twice but I don't know how to influence the counting on that level. To my knowledge there is no conflict between session dimension vs. not session dimension with "Full page url" but maybe this information is already available somewhere. Thank you in advance for taking the time.