The Filter function does not work for a hosted feature service in map viewer if the service is from an SQL Server and had the pop-up configured in ArcGIS Pro prior to publishing the map layer to Portal for ArcGIS.
This defect was logged against a version of the software which is no longer supported. The issue is not reproducible in newer versions of ArcGIS Enterprise Map Viewer. If experiencing this behavior, reach out to Esri Support Services as a new reproduction case is needed to continue investigation.