Everything is build with tolerances and each manufacturers have them different.
Some might remember Mercedes cars from 1970's and 1980's. They had central locking that was vacuum-operated.
Originally the system would hold the vacuum for several days, so even when you took vacations, all doors would still open without starting the engine.
Over the years the system would hold less, or call it leak more and days shrank to hours and then minutes.
Same on AC system. Well done would hold the vacuum for days, but poorly done will loose it in minutes.
When engine runs and supply unlimited vacuum, that is not a problem, but when you drive at WOT on gasser, you are not supplying vacuum and leaky flap system reverts to default what always is defrost.
Additional vacuum pump will do the trick, but that is more a bandage on leaky system.
Also think about dust entering the engine with those leaks.
