Labels do not display as expected in Portal for ArcGIS Map Viewer for the map image layers as they display in ArcGIS Pro when the 'Remove duplicate labels' field is set to 'Remove within fixed distance'.
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 11.1 and higher. If experiencing this behavior, reach out to Esri Support Services as a new reproduction case is needed to continue investigation.