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.