I have a zone that seems "darker"

Hi there!
Yesterday I noticed that, when I scan something, there is a zone of the scanning area that is always “darker”. Like it does not receive the same amount of IR lighting.
Since is always in the same position, in a rotating object the final effect is not so noticeable (because the part of the scanned object in this position is changing). but I think it is not normal. What do you think? Soggestions?

Here a link of a video, and a still with a detail:

dear support… no answer?
@Revopoint-Cassie … just sent an email too.
Thank you