After saving a layer in Portal for ArcGIS Map Viewer (versions 10.8.1 and 10.9) that utilizes custom symbology with either a portal-uploaded .png or .jpg file, the symbol does not load properly.
This issue is not reproducible. Refer to the following knowledge based article on how to Configure Portal for ArcGIS with custom symbology for Map Viewer Classic: https://support.esri.com/en-us/knowledge-base/how-to-configure-portal-for-arcgis-with-custom-symbolog-000012539