As long as there is some propane left in the tank, you will not have problems - but once the propane has been used, you will be left with mostly butane and that will cause loss of supply around freezing point.
If you then refilled the tank -now say half full of butane - with a 50-50 propane - butane mix, then you would only have a quarter of a tank of propane to use when temperatures fell below 0C.
Used as a fuel in a vehicle, butane is quite OK because the liquid is drawn from the tank, not the gas, and then either direct injected into the engine or evaporated using coolant heat before being used in the engine as a gas. The only time you would have problems would be if it was pure butane and the temperature was getting down to freezing, but then all you have to do is run on petrol long enough for the engine to warm up and the coolant to get hot enough to evaporate the liquid fast enough to supply the engine.
