Having a user in two separate RLS's gives the correct result in terms of filtering the fact table based on the intended security model. Because of the error caused by the December release, we have no easy way to create the same filtering result. Is there a way to enable multiple RLS per user at a report by report level? Due to the nature of relationships in Power BI, we don't have a way to Union filters.
According to the latest official document, multiple RLS for the same report user is not recommended. It's because a report user could map to multiple roles, either directly by using their user account or indirectly by security group membership. Multiple role mappings can result in unexpected outcomes. When you're considering the permission needs for a single report user, please strive to create a single role that grants all those permissions for this user. For more details, you may refer to Row-level security (RLS) guidance in Power BI Desktop - Power BI | Microsoft Docs.