I figured. I couldn’t get the stock spindle to work for me. Mine was not capable of using 1/4" bits at all. If you have a dial gauge you could try to measure how far out of round the shaft is and physically try to bend it straight. But, the bearings simply are not designed for that kind of load anyway. I have a feeling you’ll be upgrading to a router or a heavier duty spindle.
Here is an interesting thread for you: Convince me, Dewalt 611 or 400w Quiet cut? - #45 by Earwigger