I know this topic has come up quite a bit over the past couple years so bear with me. I have finally gotten around to testing this and I can publish a feature service with a related table and host it in ArcGIS Online. Because it's hosted, I noticed when attempting to configure the pop-ups in the map viewer I only get the option to see a count of my related records and only get to see the data through the attribute table. Has anyone been able to work around this? have I set something up wrong in the desktop environment? Any advice would be great.