There seems to be a bug/regression introduced in 5.1: when exporting a PNG image, the preview window in Print Mode shows the reverse page orientation of what it should when Graphics export is chosen (landscape button shows portrait mode and vice versa). The image exports correctly, but the preview is incorrect.
This is new behaviour in 5.1 - it didn’t do this previously and only started happening when I updated to 5.1. (Another new behaviour, which is good in and of itself, is that the preview correctly shows the aspect ratio of my exported image (1080p) and not the page dimensions chosen in the “Printer” tab. It’s just reversed right now with landscape/portrait.)
I made a quick screencast which demonstrates this problem. It happens on all my scores so it’s not something wrong with the one file itself.