Condensation basics
Condensation turning to ice on an aircraft outer panel would lower humidity if the system is trying to maintain 15% it will keep pumping in more moisture or fail to remove moisture above 15% to replace the water loss to the ice on the outer panel. An air conditioner de humidifier gets no ware near -50 outside temperatures. They cope with the ice because they run on and off in cycles preventing build up. When your fridge is low on gas it runs all the time and you have all seen a big block of ice on the cooling part.
Second problem is the carbon skin as a metal aircraft descends the skin will quickly heat up evaporating any water or ice in the decent stage, however a carbon skin will be like a plastic cup in a dishwasher notice anything plastic in the dishwasher is still wet when it’s finished, The natural insulation of plastic which has been used to calculate the insulation requirements means it cannot retain enough heat from the dry cycle to evaporate the water , Hence the ice melts but stays as water on the panel. Hence the rain inside on the ground. A more serious problem would be an aircraft weight and balance changing to a dangerous level. Lucky business is at the front and you can upgrade people.
A check could be made on a flight perhaps on the polar routes seeing just how much ice is building up on the outer skin inside, on a normal SLF hopefully I'm wrong or a easy solution can be found its a fine aircraft.