Same issue here with Android 10 (Oxygen OS 10.0.1.GM57AA). Web maps build in my organization do not show the Esri basemap when opened in Collector. If I switch to another Esri basemap, the basemap does load. Switching back to the initial basemap it does not load. It doesn't matter which Esri basemap I set in the web map, the default one does not work in Collector. Maybe related: When installing the app, I got the message that the app is build for an older version of Android. In addition I'm not able to download the map in Collector. I got an error saying 'Parameters cannot be null'.
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