*** Not sure if this is a known issue or not, but please reroute if needed.
I’ve created a feature layer with a one-to-many relationship to a table in ArcPro (GDB) and moved it to ArcGIS Online using this method. And it worked. In ArcGIS Online, under the hosted feature layer Data > Fields tab, I see the related table I created. I can also see the hosted feature layer and the related table in Map Viewer.

But it appears that Map Viewer does not detect the relationship between the hosted feature layer and the table. A user posted this blog which seemed to work in the past, but not anymore – the related table does not show up under the Forms tab.
It would be great to see Map Viewer work seamlessly with related tables, especially with one-to-many relationships and multiple tables.