Tough to say could be anything, loose tool/gantry/machine flex, or bad tool path
The easiest thing to try is spindle speed, if you get a poor cut heavy vibration bad dimensions it is usually too slow, too fast and you can get burning of the wood.
Powder usually means too fast, you want shreds, or small chips. Powder means you are recutting your chips because they can’t get out of the way of the bit as it comes back around so they get cut several times, giving a poor cut quality because your tool is working harder than it should. high spindle speeeds are best with a single flute if you can’t turn your speeds down. 1 cut per revolution is half the speed of 2 cuts per revolution.