How to make the .proto definition files visible in Android view in Android Studio (4.2)?

When using protobuf definitions in my gradle projects, protoc works as expected. Everything builds as it is supposed to do. However, when working in the Android view (I mean View->Project->Android), the definition file doesn't show up. On the contrary, it is visible from View->Project->Project. There I can find the my protobuf definition files as expected in e.g. app/src/main/proto.

Sources are defined in my build.gradle like this:

android {
    sourceSets {
        main {
            proto {
                srcDir '../src/main/proto'
            }
        }
    }
}

What do I have to do to make the proto source files visible in the Android view?

from How to make the .proto definition files visible in Android view in Android Studio (4.2)?