When I add a new value to a domain for a hosted feature layer then Field Maps errors with the error Domain already exists and the code 3079. Collector does not get this error.
The layer was originally uploaded from an Enterprise Geodatabase in ArcGIS Pro. I am able to modify existing domains using ArcGIS Online. I am not able to add new domains in ArcGIS Online because the button to do so does not appear. I cannot modify the domains of the Hosted feature service in ArcGIS Pro because it gives the warning "The domains for this workspace are read-only".