Thank you for the update @Ian_Ward. It looks like realm android documentation doesn’t mention to add implementation "io.realm:android-adapters:4.0.0" in the dependencies section. This fixes the above problem for me.
I’m migrating an older project that was working fine with full sync, based on the quick-start. https://docs.mongodb.com/realm/android/quick-start/
I ran into a similar issue. A ton of clean/rebuilds later I got it to work.
Seems like the final solution was updating android-adapters. I changed
implementation “io.realm:android-adapters:3.0.0”
to
implementation “io.realm:android-adapters:4.0.0”
I also still had to add these 2 imports explicitly.