Well I did those tests today and I'm still confused, as to what the 'RTH at current altitude' setting is meant to do. As far as I can tell, it has no effect.
I had the
Mavic 2 Pro hovering at 40m with the Return to Home altitude set at 20m. For several tests, the aircraft flew back to the Home Point at 40m, when 'RTH at current altitude' was both on and off. This suggests that the
M2P's default behaviour is to fly home at the higher of either the Return to Home altitude or the current altitude, which is consistent with the User Manual 2018.08 on page 18. Good. So far I have been unsuccessful in using the 'RTH at current altitude' setting to stimulate any behaviour that is different from the default.
One more test I will try is to position the aircraft in free space at say 20m, with no detectable obstacles in the sensors' field of view or range, but with a large, dense tree between it and the home point. The Return to Home altitude will be 40m. I'm hypothesising that if the aircraft starts in clear air and with 'RTH at current altitude' on, perhaps it will begin the RTH process at the current altitude, only rising when it sees the tree. Hmmmm. I think I'll choose a soft looking tree.