The only thing I can think of which might cause this problem is that the pipe does not extend high enough on the roof.  This might be similar to a chimney which undergoes a temperature inversion and the cold air falls into the chimney and causes a smell.  Perhaps the cold air is "falling" into the vent and freezing the water vapor which is rising.  There might also be a blockage in the vent system somewhere which restricts the flow and allows the cold air to fall.  A plumber should be able to investigate this system and make any repairs.

Hope this helps.