Good to know! If anyone is so inclined, I think a good follow-up experiment might be to take one of those cardboard spools, not dried, and then seal it together with a dry roll of nylon or PETG or whatever interests you. Weigh each one individually before sealing them up together. Then come back days or weeks later and see how much, if any, of the water weight shifted from the spool to the filament. Then maybe we’d know better whether it warrants sequestering the cardboard away from filaments, either via rewinding or some other means.
Speaking of weighing things, I’ll just mention this as an aside, because I hadn’t previously known about such scales: I just recently acquired this 3kg scale
https://a.co/d/9TmRFhX
and it has a nifty feature: you can do multi-point calibration on it, so that you can calibrate it with known weights at different points along its scale, to better control for non-linearity. I’m not a metrologist, so I can’t really guess as to how much of a difference this is likely to make, but after you hear about it, it’s one of those “Gee, why don’t all scales do that?” kind of things. It only requires the manufacturer to do an almost trivial amount more programming than it would otherwise do anyway.