Ready Kirk??
Hang on because here we go....
The absolute pressure in the tank is related to temperature and fuel mix exclusively. You may think it is propane, but notice the sign says "LP Gas". it is usually a mix of largely propane and with some butane and what ever else they swept of the floor. So, that is not an issue.
The regulator (whether single or two stage) should (Note: Should) deliver fuel to the appliance at a constant pressure of 11 inches of water column (W.C.) and this is referenced to local air pressure. So the absolute pressure can vary with altitude, but it will only be lower with increasing altitude. So, that is not your issue.
The burners used in gas appliances are pretty much all the same. They use the velocity of the incoming fuel to drag along the atmospheric air to make the desired fire. This is where your problem shows up. Because there are fewer pounds of air (by mass) per cubic foot at 9KMSL, the fuel needs to drag in more air to burn. It can't do that very well, so the flame front has to get farther from the burner to get to the more air that it needs to have to burn.
If some of that slipped by you, and or my explanation seems incomplete (this is very likely also), please try again and I will try to fill in what ever is missing.
Matt