Controlling humidity after a long term collapse is much more difficult. Less so in arid areas, but their hardly ideal without modern infrastructure.
As for humidity, I think you would be surprised. Much of Appalachia and the East Coast have high humidity levels (typically 70-90% during summer IIRC) and I've never found ammo damaged by humidity there. I've shot 25 year old ammo that was put in an ammo can and dumped in a hiding spot near a pond (so, very high humidity) and had no issues shooting with it. Even if the ammo is stored in the manufacturer's cardboard box, I wouldn't worry about humidity damage unless average levels are over 85 or 90%. Water damage is a different matter, but I don't have any experience with that.
Edit: It's mostly true that ammo in a fire will just pop and maybe just throw a little brass a short distance.
BUT from that fire I'm witness to the fact that ammo can also "fire" with enough force to go through steel 50 cal ammo boxes and continue through such things as walls and cans. There were bullet holes through the shop's paneled and insulated tin walls. https://www.thehighroad.org/index.php?threads/ammo-in-a-fire...
I will admit I may be overstating the humidity issues, my personal experience was 15 year old ammo in Florida shed which had issues but clearly YMMV.
Some parts of Florida are more humid than Appalachia, so maybe the difference between 75% and 95% humidity has a greater effect than 55% to 75%.
As to humidity, I don’t know it might be just extreme humidity or it could be humidity + temperature, or perhaps we had a bad case of ammo to start with.