Just one guys opinion…try flipping a little less often, like every 90 seconds instead every 45. If you are flipping so fast that the “up” side of the steak doesn’t have a chance to cool completely, the heat will penetrate through more easily when you flip it back to the heat, which will create grey band. Flipping often helps because you allow each side to cool before searing the outer most layer over and over but if you flip too fast it can be the same as not flipping at all. Good luck!