Secobdary monitor works fine in Loupe view at all magnification in Library mode. Once I swtich to Develop mode the secondary display in Loupe view will be fine for a split second and then it will become blurry. Looks like the 1:1 preview generated by the develop module during edit is not displayed properly in the secondary monitor.
I have the same problem. MacBook Pro 2019 with Big Sur, secondary monitor LG 5K, first second of displaying an image there is a sharp image then blurry/pixelated. As if it loads a small JPEG and shows it in 1:1 loupe view. Tried rebuild previews, GPU to Auto. No effect. really annoying.