I'll report the BUG to ESRIUK who can escalate up the relevant channels. But I have found that if you uncheck "Use in ArcGIS Collector" in your WebMap settings, your webmap capabilities in Field Maps swtiches to "Read-Only" when it should be "Editing".
I wanted to prevent users from accessing web maps optimised for Field Maps in Collector, but that's not possible now.
For info, I'm using Android and iOS Field Maps on multiple devices, so this isn't an isolated bug.
If you come across the same problem, you'll need to recheck the collector box, and then refresh Field Maps app. If the offending web map is your active map, then you'll need to switch to a different active map before you refresh the app, becuase the capability doesn't get reinstated on an active web map.
Solved! Go to Solution.