Actually, I believe that all who posted about a pressure delta are mostly correct.
Recall that the pressure of propane gas is a function of the vapour pressure (which is a function of pressure and temperature). This said, the pressure of the propane being released from the cylinder is regulated by a diaphragm regulator which may allow more pressure at a greater altitude due to the lower external pressure.
Those who say that the ambient O2 is the limiting factor may also be correct (in theory) though I am not sold on this one. How do you think jet planes are able to be propelled at high altitudes?
Yup - with just a glazing over of the possibilities, my vote is with the pressure difference between the propane in the cylinder and the ambient pressure.