Hello ArcGIS Online Team,
It appears that there is an equivalency issue with 'Referenced' Feature layers in Map Viewer's Editor Form builder.
When trying to set up a Map Viewer Form with related tables, 'referenced' feature services with related tables do not show up under the form's available layers when it's in the map, whereas 'hosted' related tables do show up. Below is an example for a hosted service, what we should be seeing for referenced services as well but are not:

Due to this issue, this limitation also affects the ability to edit related records in Instant apps as well since their forms are configured within this tool.
Please look into this issue as referenced services are just as heavily used as hosted services and this is an additional Esri workflow that should be supported.
Thank you,
Amanda Huber