The date format changes when editing in Portal for ArcGIS. When an edit session is started, the date field reverts to the standard US date format (mm/dd/yyyy), even if the date field in the pop-up is changed to a different format.
This issue is not reproducible in ArcGIS Enterprise 10.8.1, the earliest supported ArcGIS Enterprise version. If this issue persists, contact Esri Support Services.