I have encountered this before. It's probably the normal behavior based on how the permissions work.
The default #1 admin is a member of Registered, Administrative, and Moderating. If you set Revoke on a node for Registered then admins won't have access to that forum unless you also set an explicit Allow for one of the other two groups. This is because Revoke actually overrides an inherited Allow from another group. Only an explicit Allow can override a Revoke. I was confused by this at first so I raised the issue with the devs.
Under the current system, the better way to restrict a forum is to check the Private node box in the Node Permissions. Then explicitly allow those you want to access it (instead of restricting those you don't want to access it).