Been there done that...starting 5 years ago...only happens in the hot humid months.
Freaked me out too...now not so much...but you don't want them in your kitchen.
Has nothing to do with mold.
Freshness doesn't matter either.
Purina doesn't care, but they'll send you some coupons.
One summer they did start putting DE in the feed.
Haven't seen any yet this summer.
I now isolate my new feed bag on a tray in the garage, because bag can look fine(they wipe them down often) but within hours they will be on outside of bag again.
If I find them, I transfer feed into buckets with lots of DE(will still take a few days to kill them all) before transferring to coop bin.
https://www.backyardchickens.com/threads/grain-mites.981212/