Viewfinder "vignetting"/"tunnel view"

My vote is that the type of screen on the back of the camera is the culprit causing the darker corners of the view. The size of the screen is large and the angle of view of the corners is more oblique than through the centre, which appears brightest. The corners are darker due to increased corner illumination "falloff" due to the screen anti-reflection finish. I also note that the view of screen is a trapezoid so the shot is not perpendicular to the VF.

Here's how to verify, or rule out my take (!) on this:

If it's caused by the viewfinder screen, changing the angle of your view of the screen should change the appearance of the corners somewhat. For example, with the camera view fixed on an evenly illuminated wall, say, move your head upward and to the left. That should make the UL corner appear a bit lighter. Moving your head in the opposite direction would darken that corner.

The FIX is to hold the camera a bit further from your eye and look at your viewfinder screen square-on. That might not fix the corners; but, it may reduce the problem.

Some viewfinder screens show this darkening at the corners. It was most pronounced with deeply etched ground-glass screens in early "view" cameras and some 35mm cameras.