Myself and a number of other people lived in the deep So. USA. Between New Orleans the East coast of Florida. Especially anywhere South of I-10...
No one living in these areas will ever say there is no humidity. Each summer day it is up to 98 degrees with 98% humidity. Add on that it rains every day at 3PM. The air is steamy all day afterwords. You can see the water vapor leaving the ground. In the morning
windows and the grass and every thing else is literally dripping with dew.Some parts of the year it is so foggy at night both inland and at the coast that going outside guarantees one will be wet in minutes. The problem here is drying paint. Not blistering panels. Suppliers have plastic rolled up in the warehouses by the mile. That stuff around in storage has plenty of time to pick up water from the air. Cur panels lay around in shops for weeks and no one has a problem.
If a polycarb panel was absorbing that much water to make blisters as shown above it would be a problem in nearly every job done in every shop in every area South of I-10. Old panels, new panels,, wouldn't matter. No one has that kind of trouble. Drops laying in shops for weeks or months get used with no problems.
Then that water answer does not explain why installers have bubbles on glass installs for that matter. It does not explain pickle skin vinyl on pre-finished aluminum panels such as license plates or trailers. 99.8% of the time the real answer is a inexperienced installer making a mistake.
Yes there are a few that say different. Yes, there may be certain circumstances where a panel blistered. But for the most part that is a very rare instance in any case. So rare that only those who see unicorns would be concerned with a hygroscopic induced ruined polycarb panel. Of course that is only my opinion..