That's exactly right. At full charge, the voltage is just a little over the limit, and the JB3 will not start up. You need to pull the internal battery to unlock it once it's been give this 'overvoltage'.
When I used it, I'd charge the batteries and then take a regular 12v halogen bulb and run it on the pack for about a minute. If the JB3 powers up, you're in the range.