Oh, never mind Markus.
Anyway... this is what I've done (I hope I can explain it well):
What I've done actually was to add an existing project (with module groups) from a different solution to a new solution (major changes/new release for the same app). I've merged the repositories in this new solution and checked[updated] the [group] modules - everything was fine.
Now, in first solution I had a big Test Suite which I copy-pasted in the second solution. There is where the issue popped up. Even if the group modules were the same or as I said, I've tried to delete and create new ones with the same name, the test runner could not find them.
Today I've been reminded that the references are made by GUID, so I've fixed my new test suite by re-adding all group modules.
NOW: I guess this is not a bug, neither a feature
(I could start thinking about a request here... but let me toss my thoughts until I find something that could be really useful for other users as well)
Thanks for understanding.