Sounds to me the heavy weight plastic bag allowed oxygen inside thereby causing the stale malt. May want to consider getting some food grade buckets with decent lids for malt storage especially since it's unknown what chemicals were used to make the heavy weight plastic bag. Was the heavy weight plastic bag food grade?
BTW, if the malt is stale you'll know it. Ever had a stale cracker or pretzel?
Since the grains are hard and it tastes OK, I don't think you can conclude that it's stale. And I cn imagine a matter using a bag that isn't food grade. Have you seen anything to conclude that could be the case?
The thick plastic bag is the one that the local supplier used. Two pounds was purchased, and I had used some of the grain previously, and then sealed the bag again using a vacuum sealer, although all of the air was not removed.
As we buy grain in 55 lb bags, the grain is stored in the original bag until it is used up. Even with some grains that might approach 12 months old, never had any issues with storage.
To recap, this grain has a faint aroma of stale (rancid) peanuts. Or stale cooking oil. Very faint. But is still very crunchy, and tastes fine.