Yes this is a known limitation of the VSG25 design. There is no reconstruction filter after the DAC, so you will see DAC images and multiples of the sample rate. For modulated signals like QPSK, the amount of oversampling will determine how far out/close they are.
You can see a picture of this behavior in the production manual, section 6.5.4.
https://productbundlesdemo.kinsta.cloud/sigdownloads/VSG25/VSG25A%20User%20Manual.pdf
Andrew
