Correct, you are not using hosted layers.
Can you check a couple of things:
1. Do you have the latest Map Viewer Beta installed (there have been a few Beta releases for 10.8.1)?
2. Do you have the latest Field Maps web installed?
I'm testing with hosted feature layers. When you disable editing in the Classic Map Viewer it should mark the layer has query only just for that map. When I open that same map in the Map Viewer Beta and save it, I see it persist that setting. I'm unable to reproduce the issue so. I can try with SDE layers though I don't think it will make a difference.
Can you clarify the exact workflow? Is it this:
1. Publish new feature service with geometry editing disabled
2. Add new layer and table to a new webmap using classic map viewer
3. Disable editing on the hydrant layer
4. Save the map
5. Open map in Field Maps web
6. Open the table layer and convert the popup to a form
7. Save the map
8. Open the map in Collector/Field Maps mobile
Then you still see the collect option to collect new hydrants?
Edit: I tested with an SDE SQL Server backed layer with geometry editing turned off and cannot reproduce the issue. I'm going to need some more details to investigate any further.