At altitude, the oxygen in the air decreases. This causes problems for combustion engines. In cars, you notice this in the lack of power at high altitudes, old naturally aspirated diesel engines for example, often smoke heavily and speed is slow. You know altitude sensors from modern vehicle engines. By the data of the altitude sensor the control unit adjusts the fuel injection and optimizes it. In my experience, a modern diesel runs without problems even at high altitudes and the loss of power is still noticeable but not serious.
In principle, a diesel heater is also an combustion engine. So similar problems can occur. As far as I know, some Webasto heaters have a height sensor already built in, for others one can be fitted. Also some Eberspaecher can be fitted with an altitude sensor. Likewise with the Autoterm heaters. As far as I know, these sensor-based controls start to work from about 1,500m/ 5,000ft and the amount of fuel is adjusted. This is to prevent sooting and clogging of the combustion chamber. However, this can still often not be prevented, because the heaters only run at a low level. So this is not only an altitude problem. In my experience, the heater should also regularly run a time full load to burn the combustion chamber free or it is necessary to clean it after some operating time (at high altitude).