Hi Gianrico, once the joiner tries to merge edges that connect to more than 2 surfaces, the results can be pretty unpredictable.

There is a process at the end of joining that tries to separate out different volumes and break "non-manifold" pieces back into more normal pieces, but in this case something appears to be going wrong with that final processing.

There seems to be some kind of bug in that final processing in this case, and that is why you end up with that strange disconnected piece.

But it's not really a great case for me to focus on fixing, since the joiner just does not have a chance to process this situation properly anyway, due to the coincident edges. If you use one of the 3 methods above, it should give the joiner a chance to work.

It just doesn't have a chance to work right when you have any coincident edges in there...

Maybe I have not understood your message ... ?

- Michael