The fact that your problem is unresolved tells me that there may be a bug in one or more of the files dealing with group permissions.
I thought I was OK for over a month after the 7.5.8 upgrade until I started trying to change the permissions, which actually reverted the ones that had one after them to zeros after them in all the groups from Administrator, Global Moderator, to Moderator etc.
I am planning on opening a Support Ticket on Sunday or Monday so they can look at it.
In the meantime I am wondering if optimizing the affected tables might help.
