I just say it's not a bug, just a particular exemple.
The screenshot was very large, not only including 2 images side by side, but also a little part of the finder below the PM6+ window, in the right side of this screenshot.
And it's in this part of this screenshot image (including 2 photos + a part of the finder below) in a new PM6+ window that shows blowned highlights.
So, it's a logical interpretation of this action. No problem.