This is normal: your camera meter is trying to make the scene average out to middle gray. If a white background fills most of the frame, the camera often underexposes it, so white turns gray.

Two fixes help:

1. Set exposure manually rather than relying on automatic metering. A white backdrop usually needs more exposure than the meter suggests. One community suggestion is to meter from a middle-gray reference, lock that exposure manually, then shoot the white-background scene.

2. Post-process the RAW file. Product and stock images often need editing after capture. Increase white point / levels, brightness, and contrast so the background reaches true white without clipping the subject. Even simple auto-levels can improve it, and fine-tuning intermediate tones can remove the dingy look.

So the gray background is mostly an exposure and processing issue, not necessarily a camera problem. Shoot for a clean exposure that protects your subject, then push the background to white in editing.