selecting child terms with multiple parents, selects more than it should

Issue Summary

I have a content type where I'm using the dropbox in hierarchical select to select multiple child terms from a taxonomy vocabulary.
Simplified the Vocabulary looks like this:

The children 1, 2 and 3 have both TermA and TermB as parents - so there's only the three children and not six children.

Creating content, the user must select children, and is allowed to select multiple children.
It works fine when I'm selecting TermB -> 1 but if I then also add TermA -> 2, TermB -> 2 also get's selected.

So selected terms should look like this:

But it looks like

And if I then try to remove TermB->2 it also removes TermA->2

I think it is a bug - or do I have to create unique children for all of my parent terms ? (I have more than two parents, and they all need to have the same six children).