Sorry about this. We can get this checked. Have you tried switching the Renderer and checked if it's making any difference? Also, does this happen if you reset the Editing workspace? In case if these things make no difference please share a download link for the sample media with me so that we can test the issue at our end.
What is the Sequence setting? And did you open the Source monitor by say match-frame from the sequence rather than the bin?
If you open a clip from the bin in the Source monitor, it will show the image of the full size of the image.
If you open a clip in the Source monitor from the Timeline panel, it will show the 'full' image of the original clip but in the frame-size of the sequence that you just opened in the Source monitor ... which results in exactly what you're showing.