This is most likely normal behavior in the 6D’s optical viewfinder, not a defect in the captured image. Since you saw the same effect on another 6D, it points away from a faulty body.

The most plausible explanation from the discussion is the DSLR mirror design: the main reflex mirror is partially transmissive in the center so some light can pass through to the phase-detect AF system. That can make defocused highlight circles appear darker in the middle when viewed through the finder, especially at certain focus positions and with wide apertures. Because this affects the viewing path, not the imaging sensor path, the photo itself can look normal.

A diopter adjustment is still worth checking, but it mainly affects how clearly you see the focusing screen and AF marks; it would not usually create donut-shaped bokeh only in the finder. Likewise, changing focusing screens may alter the finder’s rendering, but based on the comparisons reported, this seems to be a characteristic of the 6D finder system rather than a repair issue.

If the images are clean, the camera is likely fine.