I'd be curious to confirm input voltage too, but the OP did mention 12A @ 240V a few times, presumably he's seeing the 240V reading from the Juicebox app, not just assuming that value. I also charge my '17 with a juicebox 40 pro, and it reads 240-249V depending on power grid load at the time. It steps down current if voltage is high, but still maintains just over 3.8kW until the final 10 minutes of the charge cycle where it tapers down to 1.4kW. The car will not allow above 16A if the voltage is lower, e.g. 208V or 220V. The ramp-up time is very fast on initial plug in, up to full rate in just a few seconds. I would see what their tech support has to say about it, I've worked with them a few times and they were fairly knowledgeable and responsive. It sounds like potentially a problem internal to the juicebox, not stepping up past 12A regardless of your setting, yet correctly cutting it back when you specify 10A.
A few others mentioned it, but 16A (3.84kW) is the correct L2 max rate for a '16-17 Gen 2 car. L1 max is 12A (1.44kW @ 120V), with the default being 8A, configurable in the center display. That setting does not apply to L2 charging, which is always 16A capable, at any current. The other poster's 3.3 and 3.6 kW assumptions are based on Gen 1 cars or other misinformation.