1. Create an account with the primary usergroup as Registered Users
2. Make that account's secondary usergroup Moderators
3. Set up a private forum called "Staff Forum", allowing only the moderator usergroup to view it under Applications > Node Permissions > Staff Forum > Moderators > View Node > Allow.
4. You can additionally edit the specific account to allow the user to view the forum from under Applications > Node Permissions > Staff Forum using the field at the top right. (Bug reproduced both with and without this step).
5. Log in as this user and you will notice that the account cannot view the private staff forum, despite being a moderator through the secondary account.
However, if you make the account's primary usergroup as Moderator then you will be able to see the staff forum. Surely though, the account should still be able to view the forum by inheriting the moderator privileges from the secondary account.