It's definitely not a myth; jacket separation happens. Does it happen every time you shoot light loads in a 1:7? Of course not; but it still happens, and the best way to avoid it happening is to use bullets of an appropriate weight. Also, over-stabilization effects accuracy, especially at longer ranges. Is it as bad as an under-stabilized bullet? No. You might not even notice the difference at shorter ranges. But shooting very light bullets in a 1:7 is not ideal.
0331: "Accuracy by volume."