We have multiple interfaces which inherit from another

interface ApiInterface : AuthMobileApiInterface, DashboardApiInterface {

    ...

    @GET("rest/user/context")
    fun getUserContext(): Call<UserContext>

    ...

}

AuthMobileApiInterface and DashboardApiInterface are containing methods with retrofit annotations as well.

We also have a class DemoApi which implements ApiInterface but as the name suggests is only used while in demo mode.

While upgrading to Gradle 8.4 and AGP 8.3.2 we experienced a ClassCastException ->
retrofit2.DefaultCallAdapterFactory$ExecutorCallbackCall cannot be cast to DemoApi.getUserContext.Call.
The cast exception itself makes sense because ExecutorCallbackCall implements Call but our DemoApi only provides a Call implementation.

The thing that makes no sense is that the application used our DemoApi everywhere where we used the ApiInterface instead of the service instance created via retrofit.create().
This happened with minification enabled only.
After playing around with ProGuard rules we found out that annotating all involved interfaces with @Keep resolved the issue.

I want to understand R8 and retrofit.
My questions are:

• Is this a retrofit bug?
• Did we missed something of the retrofit documentation?
• Is annotating all interfaces with @Keep a valid solution or did we messed up something?

Any guidance to understand the situation is welcome.

Facts

• Gradle 8.4
• AGP 8.3.2
• retrofit 2.11.0
• android.enableR8.fullMode=false

References

• Upgrading Gradle from 7.4.2 to Gradle 8.0.0 gives error on release app – java.lang.ClassCastException