What's the theory(ies) on piezo crystals at elevation? We still strike them, it still deforms and it still bursts electrons, right? Is it temp or altitude? Calibration of the air-gap? Odd.
It's not the piezo, it's all down to Minimum Ignition Energy (MIE) of the gas. This is the amount of energy required to raise the air/fuel mixture to a sufficient temperature for the gas to burn. The MIE varies with temperature, pressure and air/fuel mixture. The graph below shows how the MIE for Butane varies with air/fuel mixture. The lowest MIE is 0.26mJ (at sea level and 25C) and rises sharply as the mixture deviates from the optimum.
When you gain altitude, both the temperature and atmospheric pressure drop. Pressure at 10,000' is approx 70% of sea level pressure. Reducing temperature and pressure both increase the MIE - the relation with pressure is approximately inverse square law. Not only that, but as the jet in the stove and lighter is fixed, the air/fuel mixture will be richer, so the MIE will move up the right-hand side of the graph above.
The spark from a piezo has a fixed amount of energy, so there will be an altitude above which the spark energy is insufficient to ignite the gas from either the lighter or stove. A 'flint' lighter will also have an altitude limit, but I'm guessing that the energy in a hot chip of ferrocerium is greater than that of a piezo spark.