They are pretty ubiquitous in the summer, and don't cause that much of a problem other than the creepout factor, tho you don't want them getting into your kitchen!! Flipped me out at first, but have learned to live with it.
Here's from when I first found them, with video and pics.
https://www.backyardchickens.com/threads/grain-mites.981212/
Yep, see it in the Purina FR almost every summer, this year was pretty bad. One year they started adding DE to the feed and it took care of it. I bring bag home and set it on a large dog crate tray, then transfer to clean container while mixing in copious amounts of DE, takes a couple day to kill them all.
'Brand new' in that you just bought it, but what was the mfg date?
Older the feed, the more the grain mites proliferate in there.
If you call Purina with mfg code they'll send you coupons.
That doesn't do squat, TSC wipes them with a pretty strong percentage pyrethrin, asked them to do it so saw it first hand, next day bag was again covered. The grain mites are
inside the bag and residual insecticide doesn't seem to kill the,.
They come from inside the bags, but yes, can transfer to other bags.
TSC manager told me that Purina told him they were 'bag mites that ate the paper'.
Not correct of course. ETA...I told him 'those bags are plastic' and the light went on.