I was just playing around with a mirror prop, when I spotted a difference in the renders of version 1.16 to 1.17.
So the mirror it self has a frame around it. The mat zones are left-right bars and top-bottom bars. While the side bars seem to render fine, the lower part of top and bottom bars are rendering in two different shades, as if they're two parts.
This happened in version 1.17
(next post will have a picture of version 1.16)