I am not sure exactly why, but the Form I built is gone in the Field Maps configure pane for the layer, and so is the "plus" button in the Field Maps app. This has happened before, but I am at a loss to pinpoint the before/after scenario.
I did update the popup in the beta map viewer, and I did add a field to the layer - but there is no indication of why the whole Form is gone as if I hadn't built one.
Once I added the new field to the feature item definitions for the layer, I could add/edit features in the web beta, but there is stlll no Plus button on the app.
To my knowledge no one used the layer in the non-beta map viewer.