[White background prints slightly off-white]

I've tried removing the rectangle, making it transparent, saving it as RGB and CMYK. I'm baffled. I am self- taught and not a ton of experience so I'm not sure what I am missing. Most things have been fairly intuitive or solvable with Google but I'm hitting a wall with this.

[White background prints slightly off-white]

I'm new to Affinity Designer and relatively new to teaching myself graphic design. I do a lot of simple black text on white background art prints and when I did them in Illustrator the white background was true white with no discernible difference from the background of the print and the white paper it was printed on. In AD I create a white rectangle the size of my background  (like I always did before AD) with color changed to white but I get this dingy looking background no matter how I save it. I am lost as to what I am doing wrong and feel like I must be missing something obvious, and have a customer waiting on a print because I cannot figure this out. I've tried attaching a photo of the printed piece but it keeps saying unable to upload.