The grey box is likely a low opacity layer that the original creator used as part of the design, and simply didn't notice it before submitting it
Your signature has more issues than just the grey border though
It is 400x170, which is illegal in height (maximum of 150) and is ~170 kb, which is 100 kb over the limit.
Since it's a popout, shrinking the signature would be better than cropping out 20 pixels - it's quality loss vs having a flat line, and the flat line would look worse on a popout
Quote (ACMGi @ Jun 19 2021 10:22pm)
And never pay anyone for an illegal signature or an improper fix
The best approach would have been to approach the person who made the sig for you about the changes for an easier and likely higher quality fix