Per version 1.16 of the manual regards Failsafe Function, Sec 11.4 ...
If you are flying at less than 20m (66 ft) above the recorded home point altitude, the craft will ascend to 20m, return directly to a point above the home point, hover for 15 seconds, and then land.
If you are flying at more than 20m above the recorded home point altitude, the craft will maintain its current altitude, return directly to a point above the home point, hover for 15 seconds, and then land.
Note that GPS lock is critical in RTH because ...
If at any time during the RTH sequence less than 6 GPS satellites are locked in, the craft will descend wherever it is at that time.
If the craft is not in GPS mode and RTH is initiated, the craft will descend wherever it is at that time.
There is nothing in the manual to suggest the craft adds 20m (66 ft) to any altitude other than that of the home point. It returns at either 20m above the home point altitude, or at the altitude it's flying if greater than 20m above the home point.
This would imply that if you set the home point on a canyon ridge and then fly down into the canyon and RTH initiates, the craft will ascend to 20 m above the canyon ridge before returning home.