Generic MonoBehaviour with nested class serialization issue

This serializes:

 public class GenericMB<T, V> : MonoBehaviour
 {    
     public T  t;  
     public V  v;
 }
   
 public class ThisSerializes : GenericMB<int, string>
 {   
 }

i.e. if I have a GO with ThisSerializes attached to it, 't' and 'v' will show up in the inspector.

This works well too:

 public class KvpMB : MonoBehaviour
 {
     public KVP kvp = new KVP();
 
     [Serializable]
     public class KVP : GenericKeyValue<int, string>
     {
     }
 }

 public class GenericKeyValue< TKey, TValue >
 {
     public TKey key;
     public TValue value;
 }
 
 public class ThisSerializesToo : KvpMB
 {
 }

Howover; not the case here:

 public class GenericMB<T, V> : MonoBehaviour
 {
     public KVP kvp = new KVP();
 
     [Serializable]
     public class KVP : GenericKeyValue<T, V>
     {
     }
 }
 
 public class GenericKeyValue< TKey, TValue >
 {
     public TKey key;
     public TValue value;
 }
 
 public class ThisDoesNotSerialize : GenericMB<int, string>
 {
 }

Attaching ThisDoesNotSerialize to a GO 'kvp' doesn't serialize thus not visible in the inspector.

I don't get why; I'm not attaching a generic MonoBehaviour; It's just that I'm inheriting from one. It serialized in the first case, why doesn't it serialize in this one as well?

Any ideas?

Thanks.