This is happening with offline mode as well.
When using a map in Collector version 20.2 on an Android tablet (Samsung Tab S2) I intermittently get the following error.
Untrusted Host
https://services.arcgisonline.com/ArcGIS/rest/services/World_Topo_Map/MapServer
This server could not prove itself; its security certificate is not trusted by your OS. Would you like to continue anyway?
To close the error I have to tell it to trust it about 20 times. Then a few minutes later the error will pop up again. I also got this error on my Motorola phone on Android 10. Some of my students also get the error with this map.