The problem you have there is that you are not using a false fence, as the others have said, but you also have no support
when you have taken away the flat edge, at the end of the cut, as evidenced by you're pic, very dangerous you naughty man!
You must fix a packing piece on the false fence to stop the stock "twisting" into the cut.
I would have a piece of packer,tacked to the out feed side of the false fence, just a little higher than the bottom round,
And thick enough to keep from tilting,
In my days on the spindle, the test cut would be made with the packing already in place.
Easier for us as we had literally hundreds, if not thousands of 15" long patterns of every type of mould, and adding every year.
HTH Regards Rodders