Show Related Records in the Portal for ArcGIS Map Viewer 10.7.1 pop-ups do not adhere to the configuration of thousand separators and decimal places of a related table.
This defect is logged against a version of the software that is no longer supported. The issue is not reproducible in newer versions of ArcGIS Enterprise Map Viewer 10.9 and higher. If still experiencing this behavior, reach out to Esri Support Services as a new reproduction case is needed to continue the investigation.