I remember the powder soak up humidity myth. There may be some strange powders that do this, but it’s largely false. Many powders are processed in water as I understand it. My only moisture concern would be with the metallic components of the round. I’ve seen a swamp cooler ruin .223 brass… the powder was fine.
Someone emailed the ammo manufacturers years back on tumbling rounds and the dangers of burn rate alteration. At least one came back with the answer that powders are extensively blended during manufacturing, and that the normal handling of ammo, tumbling, bouncing around in cars/holsters, etc would never be enough to worry about.