@chrisqck , See my same issue at Bug Report: ldap user sync to skip invalid user id entry, not halting. IMHO, this is a bug. True that id in CN field should not have space, special character. The save and test will trigger the AD connection AND initiate a user dump even one clicked on “create user account on first login” .
Also when invalid user id in CN field encountered, the user dump should failed and yet it procceded to declare it was successful. BUT I love this bug, since my valid CN id can still login 