UPDATE
As I had to realize today, the possible workaround does not work for my use case.

The intention of the query
SELECT p.id, size(descendants) from Parent p left outer join p.children descendants group by p.id
is to retrieve all parents and the number of their children, if any.

But with the workaround, which does a second join, this doesn't work.
Only those parents with at least one children are returned.

So this bug is a serious show stopper for us.
We can not migrate to Hibernate 5 until this is fixed.
Therefore I'd suggest to increase the priority.

Are there any new insights about the cause and how to fix it?