No __typename included for interface fields without "onImplementation()"

[martinvisser]

In our graph we have multiple interfaces and implementations. If someone constructs a query as follows __typename is of course added during serialization:

        var query = new ProfileProjectionRoot<>();
        var account = query.account();
        account.customer().__typename().id();
        query.agreements().__typename().number();

Both customer and agreements are interfaces.

When using this, __typename is also included:

        var query = new ProfileProjectionRoot<>();
        var account = query.account();
        account.customer().onSomeCustomerImplementation().id();
        query.agreements().onAgreementImplementation().number();

But, when using fields that are defined on the interface itself, __typename is not automatically included:

        var query = new ProfileProjectionRoot<>();
        var account = query.account();
        account.customer().id();
        query.agreements().number();

Is this expected behaviour of the serialization step, or might there be a way to automatically include __typename is an interface field is queried?