Dji's explanation to my inquiry pasted below...doesn't offer an explanation of why it didn't happen as it should have. New drone is ordered, two actually, headed to China in a couple weeks. Putting one in carry on and shipping one DHL so I know I have at least one when I get there. Hope so at least....
Dear Bob,
Thank you for contacting DJI technical support.
We appreciate the time and effort you spent in reaching us. We are sorry to hear about what happened to your aircraft.
To answer your question, the procedure of RTH is that it will ascend to a preset altitude then flies to the home point then descends if the aircraft is further than 20 m in distance.
If the aircraft is flying at a height of 2.5 m or less and is within 5-20 m (not further than 20 m), it will not ascend but will fly to the home point at it's current height.
If the unit is less than 5 m in distance regardless of the height, it will just land.
For reference you can check it on the manual on page 14:
https://dl.djicdn.com/downloads/Mavic Air/20180525/Mavic Air User Manual v1.2.pdf
If the unit is within the 12 months warranty, you can request for a data analysis to further check what happened on the incident and for warranty service.
Since the gimbal has issues and it may have a water damage, I suggest sending the unit in for diagnosis. If you are in United States, you can use our online repair request system to create a case and generate the shipping label together with the case ID. Please follow the steps below: