I think the very strange behaviour was due to the fact that I did not have a name for the
gid in AD . As a workaround, I removed the gid from override (and let IPA generate one) .
The interesting part was that getent did assign the username to the respective gid
(therefore both getent group commands were successful ).
I do not know if there is other alternative apart from adding a name for all GIDs in
Active Directory in order to have the gid override working properly (if I define a posix
group in IPA for the AD trust group does not work).