If you ever found the labeling not working on arcgis online, web map viewer, web map, arcgis portal,
it is because of maplex label engine was not turn on, not enabled.
All ESRI web base map, use maplex label engine behind the scene. Even ESRI claimed old ESRI standard label engine also works with web map labeling, however, this is not the truth. In reality, even ESRI started to move from its own standard label engine to Maplex label engine at ArcPro.
ArcMap use ESRI standard label engine as cefault.
ArcPro use Maplex label engine as default.
If you ArcPro project is imported from old ArcMap .mxd, you must manually enable Maplex label engine to meet the ArcPro standard, to fix the problem such as labeling was not working in down stream web map on Arcgis online ro Arcgis portal.
