On the square wave the distortion disappears at 500kHz, 800, 1Mhz,1.6,2.0, 2.5, 4, 5, 8, 10, 12.5, 16, 20 and 25MHz.
However, the output of the arbitrary waveform is even worse it varies by 50% and the frequency is all over the place not just the amplitude. See attached.
Built in waveform Sinc.