Are you sure?
If you have an RTH height set to 50-metres for example, that is 50-metres above takeoff point. If you were then flying 50-metres below takeoff point and hit RTH the drone will climb 100-metres before starting its return home.
Your drone does not know what sea level is.
It only knows that home = 0 feet
No ... it makes no difference whether the height is + or -.
How so?
It shouldn't make any difference at all.