Without even looking into this.
I can only assume it is the permissions with the greater than less than settings where a member is in multiple groups.
And the -1 does not work due to a logical compare to -1, 0 and 1.
It has caused some heartache for me in the past.
Usually most of it comes from a new site user where they just start using it.
And to fix most of it , not all . Means completely restructuring the groups.
Which after a while on a large board nobody wants to go through it all.
