Its not altitude, ive flown both generations of mavic at significantly over that height and all is fine.
From my experience boats a few things are more likely. If there is a lot of movement or roll on startup the IMU complains of errors during initialising and gives a "cannot take off" until things have calmed down. Its quite possible a balloon isnt stable enough for it.
Ive also heard but not personally seen that motors wont start if its moving too quickly. If there is movement, as soon as it takes to the air its going to go full power in the opposite direction to station keep and likely hit something.
Ultimately, from a platform like a balloon, ATTI mode is likely to be the only sane method, especially if you want it back in the basket later and not on the DZ at ground level.