If your password policy is set for something tough, such as non repeating passwords 341035, an admin goes in and resets the password to 12345. Then notifies the subscriber the pin has been reset to 12345, it really was not reset. Unity will accept this in the admin, but for some reason, does not always flag this as invalid pin for the tui.
Check your pin complexity settings first, then create a new pin that matches the requirments.
I have had this happen many times with newer admins.