Find the user that can't see the event in acp and see if you can find any differences? maybe these accounts are not added correctly?
Create an event with this username and check if you can see it?
This permission system is great because ou can do just about anything you like ... it's very complicated for the same reason
I just now was very happy to have a very recent backup because I made a few changes to find the reason of your problem (all of the sudden I seemed to have promoted any user to moderator)