I would love this feature to work, but it works the exact same way for me. I think they removed this functionality a year ago or longer.I am unable to adjust my trunk height in my garage. I too have scars on my forehead from the trunk edge. Supposedly, you can reset the GPS fence by manually raising the trunk above the sensed level (there is plenty of room), and then pushing and holding the trunk close button until it beeps. Mine beeps just fine, but then when you next raise the trunk, it stops at the previous lower opening (ignoring my requested change). Repeating doesn't help. I called Tesla, but they have not done anything.
What you can do is force the FWD doors to open all the way on the big touch screen and have it remember that because it will also apply to the trunk. This is what I have done.
The interesting thing is that when my garage door is closed, my trunk now opens to full height. When my garage door is open, it actually opens a little lower. This functionality seems to be constantly changing...