Currently choosing a specific implementation for a SerializeReference interface fields in the inspector is not supported. To achieve this users will need to implement custom editors for these types.
It is however confusing that user could select implementation for Example1 as shown in the screenshot, as it shouldn't have been possible and we were not able to reproduce it. If this is the case, please feel free to reopen the bug for further investigation. In doing so, please provide more details such as the Unity version used, information whether any custom editor or packages were used.