Filter builder in Tabbed list does not work in Service Portal when same table is used in different tabs. Let's say there is a Service Portal page which has two Incident lists in two different tabs and filter builder is enabled. In this case, the filter builderin the first tab will work but not the other.
This is an expected behavior as per the design. The filter builder won't work in the other tabs if the same table is used.
Also, if a user wants to apply filters on the records of the same table they can apply it on the first tab and get the expected results (number of records). So, it doesn't make much difference if the filter is applied on the first tab or the other as long as they point to the same table. The purpose is served in this case as the user can see the records as per the filters. It makes more sense for the filter builder if different tabs point to different tables (For example, Incident table and Request table).
Refer to the community article on Service Portal - Tabbed List Widget