I was able to set up a role product that granted the role when the order was completed. The credit card was processed through the Test gateway, and a Workflow-ng configuration updated the status when payment was received. The only difference I can see was that the condition about the balance was set to be equal to $0 instead of less than or equal. I don't really think that would cause it to not work, though.

Sorry, but I can't duplicate this bug.