I just uploaded the flight log (621fb61). It seems to have behaved exactly as I thought. You can see the RTH activated many times and the drone just kept getting farther and farther away. Bummer not to expect some type of refund from DJI but I guess I understand. With electronics, stuff happens.
Phantoms don't usually just fly away and I expect the same is true for the Mavic.
Looking at your flight log, I see some parts that don't quite match your story.
But that's OK, confusion is common and that's why we have recorded data..
I think this flight will be one of those painful lessons that is very effective because it will cost you.
The cause appears quite obvious.
What can you say about the wind conditions that day?
After a few minutes of good flight, the Mavic Pro lost connection and just flew away.
The flight record does not agree with this.
The Mavic maintained connection and was responding to your controls the whole time.
The Mavic suddenly rose to over 900 feet in the air (despite my RTH height being set to 100 meters) and just flew away and was non-responsive to manual controls.
There have been cases of Phantoms and Mavics climbing when they were pointing into the sun and obstacle avoidance was to blame because it kept climbing to avoid hitting what it thought was an obstacle.
But I don't think that it the case in this incident.
Looking at the flight record, it shows the left stick being pushed hard forward from 9:39.2 - 10:42.5 and again from 11:03.9 - 11:05.2.
That is the cause of it climbing to 935 feet.
and just flew away and was non-responsive to manual controls. I hit the RTH button but no response. I tried the combination of manual flight and RTH five times but soon it was nearly 3,000 feet away an over the water.
From 11:12.6 you brought the Mavic back down and closer by using full left and right sticks.
When you eased up on the right stick your speed quickly slowed.
When you tried Sport mode, with full right stick, the speed went up to 20 mph and you were getting closer.
You brought it down to 432 feet and only 387 feet away at 12:45.5 when you engaged RTH mode.
You left the Mavic at 432 feet from then on.
In RTH with no stick input, the Mavic is going backwards at 2-9 mph
From 13:49.8 you mysteriously pulled the right stick hard down sending the Mavic backwards at greater speed and quickly lost another 300 feet distance.
And from then on you didn't touch the right stick letting the Mavic be blown away.
Some points to remember to prevent this sort of thing.
The wind is always stronger up high.
If your drone is struggling against a headwind, bring it down lower where the wind is less.
RTH is a slow driver. If RTH isn't making headway against a wind, take over and do the driving yourself.
You can fly 50% faster (in still air) than RTH.
Your Mavic didn't just fly away, you allowed it to be blown away.
It should have been a simple matter to bring the Mavic back when it was only 387 feet away at 12:45.5.
Simply descending would have made it easy.
If you look back earlier in the flight from 70% - 60% you flew almost the same direction at 104 ft and could travel at 17 mph with full right stick.