I think very few manufacturers extensively test the feed stock pellet when they come in, and maintain a rigid control over storage and handling. I have had a particular batch (color) from one supplier have this humidity problem right from the vacuum package, while six other colors (also batches I presume) were perfectly fine. I have also had very expensive high quality european made PETG filamant have this exact same problem.

The fact that you haven't tells that you either purchased from another source or just have had pot luck with your purchases. I go through a lot of ilament and have used up / am using about 20-30 kg of PETG alone in the past 6 months. So far one color from a chinese and all PETG from european source were problematic.

And again, when exposed to humid atmospheres, all polymers will take up water to some extent. How much depends on the type of polymer, the ambient temperature and the ambient absolute humidity expressed in grams of water per cubic meter of air. Forget relative humidity, it is meaningless (unless stated together with the temperature, so youcan calculte the absolute humidity from it) when it comes to gas-solid or gas-liquid equilibria.

I have had spools of PLA, ABS and nylon which were either too water-rich after manufacturing, or developed this issue after some time being exposed to the ambient. Regardless of source. It is just a physical characteristic of the polymer and can't be helped other than air-tight storage and drying when appropiate. If the manufacturer would thoroughly test each and every batch, and safeguard the handling an storage of the feedstock before it is being used that would solve the out-of-the-box problem (which, as a chemical engineer and knowledgeable of how batch manufacturing processes work, I do not think is done in practice). However, the ambient exposure problem remains.

In central heated conditions (say 20C / 35% RH = 6 grams H2O/m3 absolute humidity) I can leave my spools of filament out open in my workshop for weeks. However with hot humid weather(say 35C / 95% RH = 38 grams H2O/m3) the same spools absorb water in a matter of days to the point I have to vacuum dry them to be useful again.

Having said all the above, unless specifically quoted to be european or US made, most 'brand' filaments originate from the same manufacturing sites in China I suppose. If the box does not specifically say 'made in USA' or 'made in Europe' I would bet my money on chinese origins. No problem, the chinese make excellent stuff as my laptop, smartphone, smart TV's, (actually most electronics) etc all can testify.

As to printed PETG being in the weather, frost will not harm it as the water molecules are so dispersed throughout the polymer and can not form ice crystals. When heating however, as in the nozzle, the water turns to steam which is a gas. This boils out of the polymer. Cold is not a problem, heat definitely is. Unless of course there are pockets of water in gaps in the printed object, which will form ice when freezing and thus expand and possibly crack the plastic. But that is a macroscopic issue, not one on a moleculer level.