Hi all,
I have a Feature Service in Portal (Referenced/Non-hosted) that I add to ArcGIS Online as a service. There are two Feature Layers that have a relationship of 1:M.
When I add the AGOL Feature Service to ArcGIS Pro I can click on a feature in Layer_1 and see the related info in Layer_2 in the popup.
When I open the Feature Service in a Map Viewer in AGOL there is no option in the Popup to add Related Records like you would normally see as per below. The Related records option does not display.

If I open the Feature Service in Map Viewer Classic, the Show Related Records is available in the Popup.

The Relationship is definitely in there but the new Map Viewer will not pick it up.
Now, if I publish as a Hosted Feature Service to AGOL (which we do not want to do), the ability to add Related Records to the Popup works.
Is there any known issue with taking a Service from Portal and adding to AGOL and not being able to use the Popup > Related records functionality?
Any insights greatly appreciated. Please let me know if you require any further information.
Some details:
ArcGIS Enterprise version 11.1
Known limitation with Map Viewer 11.1 that there is no option for popups to display Related Records.
I know I can use Arcade and work some magic, but that is another level of maintenance that we would like to avoid if possible.
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