Hi, Jean!
The paradox is that an entity can have two parents in such situations.
And it’s like with many to many - a lot of questions rendering. By the way, we can formulate the limitations of our hierarchies as follows: there is only one parent for an entity. This also explains why only the top type can be self-nested.
Some cases are also limited by our lack of polymorphic relations, as far as I can see your child Entity can have only one parent at the same time, but there is a possibility for another one as well, so we treat it like it has (can) two parents at the same time.
We understand that such cases were pretty common, but we just don’t have a good solution in mind. 