It does not ascend to 10 meters and then return, it begins the ascent as it moves towards home. If it is not far from the home position, you might not notice much ascent.
Can you describe more or what you saw:
How high were you before you initiated Home mode?
How far from home was it?
Did you see any increase in altitude at all? O see any indication of ascent on the ST-12 screen?
I tried at a few different heights from 10-20 feet. Toggled to home when about 30 -50 feet away. May just a subtle change in height but not much. I recall that the one i had a couple years ago would ascend when this close to home.
It's too dark outside to do an experiment tonight. I will be away from home most of tomorrow. I seldom use "Return to Home", and I've always been at altitudes above 100 ft when I've done it with a Q500, so at the moment I can't make a reasonable statement of "normal" for Q500. I did do RTH on the predecessor of the Q500, a 350QX, which has very similar mainboard and programming. That went up at about the same rate as forward motion. Nothing subtle about it. It will be late tomorrow, but I'll at least be able to let you know if another Q500 matches yours, or confirms your suspicions. Not sure if RTH is using GPS for it's altitude, or if it's using the altimeter. Both have a pretty bad accuracy, so maybe? But I kind of doubt it's an accuracy issue. It sounds like you did several trial runs with consistent results. It may be more of a calibration issue, and calibrating the altimeter on a Q500 is NOT a normal "Home Remedy".
One thing you can check is the altitude that is displayed on screen. IF the Q500 THINKS it is above 10 meters (33ft), it will just maintain it's current altitude. And that much error is pretty common for a Q500.
I checked the User Manual for the Q500 and the action of the Home mode is that at altitudes above 10 meters the aircraft will maintain its altitude and fly straight back to the controller and land from 13 to 26 feet away. If it is below 10 meters it will gradually gain altitude up to 10 meters as it flies back to the controller and land as stated before.
When the Q or H series is close to the home point (controller) it will not ascend. I don't recall the distance but I believe it has to be over 100' away, perhaps more, before it will ascend to the RTH altitude.