Quote Originally Posted by 2qkv8 View Post
Turns out that 'Relaxed User Matching' is another option for a
workaround. It doesn't clean the NCA user population from all the
unnecessary accounts from deleted Identity Source, though. If you wish
to wipe them off completely the only possibility without NetIQ support
assistance is to deploy the environment from scratch.

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That was my guess as well, thanx for reporting back!