Primers are pretty well immune to a lot of things. Both the Navy and Norma have done powder cooking experiments and can deteriorate it badly with heat at about 140°F, but it still takes a year or two staying at that temperature. Primers seem unaffected by it.
Humidity and powder do interact, though. Norma's manual shows that powder stored in 80% RH burns about 12% slower than the same lot stored at near 0% RH. They also show that even the powder in a loaded cartridge gradually assumes the environmental humidity level over a period of about a year. I expect something similar for powders stored in containers that have been opened. The reason it happens is that water molecules are the smallest molecules known. At about 0.275 nanometers, they are about 300 times smaller than the polished surface finish on of a mirror. Most of us don't get our brass that smooth in the necks and most of us don't screw a lid on tight enough to weld it to the container. Now you know why the military likes sealants.
In addition, modern powders are sold in plastic jars rather than metal tins. Plastic is not nearly as long-term moisture-proof as metal. All plastic resins have finite water permeation rates you can look up, so eventually moisture gets through them, whether coming or going. Imperfect lid sealing probably allows rather more. So eventually the powder reaches moisture equilibrium with the environment you store it in, even in a closed or sealed container. Only the old soldered hermetic military ammunition tins stood a chance of holding ammunition to a constant humidity level.
