Maybe this shows why they make their own Programmer readily available ?
It may be related to calibrate, and doing that on a Serial link can be tricky.

eg I've sent continual bursts of a known char to get high precision calibrate on other MCUs running my own code, and I've seen some systems increment the baud in small % changes.

All that means a change in buffer size, or baud granularity, or even driver, just might make the difference.