Pine lumber can definitely vary significantly in quality, but I’ve not experienced the kind of change you’re describing.
The 1x6s I generally use can have all kinds of cupping, warp, twist, etc in the stack at the store. Generally if it is one of the “reasonably” straight and flat when I buy it, it stays that way.
The “rejects” typically have the largest growth rings and have a well defined “rainbow”.
When I buy a typical load of 40 boards, I spend quite a while sifting through the bunk for the good ones. Sometimes, if there is a fresh pile, I’ll buy more than normal to try and make sure I don’t have to come back when there is nothing but crap left.