Have created a map in our AGOL tenant using a registered feature service from ArcGIS Server 10.6.1. Popup definitions created in the new non-beta map viewer don't seem to be used by Field Maps. Popup definitions created with the "classic map viewer" do seem to work. Why?
Hmmm....cross posting to this question. I can confirm that the reason is a lack of capability between popups in the new map viewer and Fieldmaps. I had to use the "classic" map viewer to config my popups. This then displays correctly in Fieldmaps.
Please check out the latest release of Field Maps (Nov 2022) - where we have added support for popups configured with the (new) Map Viewer.
See the "What's New in Field Maps" blog for all the details.