Steps to reproduce:

Mark an existent method as default;
run updateApi;
method that was marked as default disappears from existing classes that implement that interface;

For example in ViewModelProvider.Factory mark a method create as default:

@NonNull
default <T extends ViewModel> T create(@NonNull Class<T> modelClass) {
  throw new UnsupportedOperationException();
}

then run ./gradlew :lifec:lifec-view:updateApi

It will fail with:

/Users/sergeyv/android/androidx-main/frameworks/support/lifecycle/lifecycle-viewmodel/api/2.3.0-beta01.txt:37: error: Removed method androidx.lifecycle.ViewModelProvider.NewInstanceFactory.create(Class<T>) [RemovedMethod]/Users/sergeyv/android/androidx-main/frameworks/support/lifecycle/lifecycle-viewmodel/api/restricted_2.3.0-beta01.txt:37: error: Removed method androidx.lifecycle.ViewModelProvider.NewInstanceFactory.create(Class<T>) [RemovedMethod]
Aborting: Found compatibility problems checking the public API (/Users/sergeyv/android/androidx-main/out/androidx/lifecycle/lifecycle-viewmodel/build/api/current.txt) against the API in /Users/sergeyv/android/androidx-main/frameworks/support/lifecycle/lifecycle-viewmodel/api/2.3.0-beta01.txt