I deleted my previous reply because I played with things a bit more, so while what I said in that reply was true for a constant zoom level, it isn't if one zooms in and out on the same image.
There seem to be two versions of a preview image, one that has smoother and one that is sharper. The smooth one shows up, initially, and may be a video-driver resize of the previous preview, whatever size that was, then the sharper one shows up after LR has recomputed it from scratch.
I am not sure I see the same things you do at 1:2 zoom when switching back and forth between Library and Develop--the two images appear the same to me, pixel-for-pixel, once I have clicked back and forth between Library and Develop a couple times so that the preview is being retrieved from something computed accurate at that size rather than inaccurately resized from another size as seems to occur the first time I click into Library or Develop at a new zoom level for a particular image.
The sharper one at 1:1 zoom seems to match the Exported one, now, and so that is the accurate one. I think you're calling this accurate one the worse one because it is lighter and shows more noise. I think this is just because Adobe is actually applying all the noise-reduction and sharpening to the Exported image before resizing it smaller and sharpening it with the Export sharpening of Screen-High that you have specified, now.
Before what was happening when you first produced your example JPGs that are in the ZIP using an earlier version of LR that had a bug, was is that LR wasn't applying the sharpening to the image if it was smaller than a certain ratio compared to the original, and the first two of your three images were larger, and were being resized down smaller than that ratio and didn't have sharpening applied and looked smoother and darker, but this was inaccurate. Only the last image where the original was cropped the smallest was resized down still large enough to have the sharpening applied, so it appeared lighter and has more noise. In other words what I'm seeing with your original JPGs was two darker-smoother exports and one lighter-rougher one that is more accurate.
Then Adobe fixed this ignoring of sharpening in LR 5.3, but the Develop preview, when resized smaller than 1:1, especially non-whole-number-zoom amounts used with Fit and Fill, seemed to accentuate noise by using a simple, fast, and somewhat inaccurate resampling algorithm. This caused confusion because the now correct Exports were smoother than the overly-noisy Develop image if resized smaller than 1:1.
With LR 5.4 I think the Exports are all the same brightness because the Detail and Sharpening settings are being applied consistently starting in 5.3, and Adobe has made the Develop previews more accurate, with a more time-consuming re-render of them at a smaller-than 1:1 zoom. Or at least this is what I'm seeing, because for me the Develop previews don't change in noise and brightness between Library and Develop other than momentarily in Develop before things become the same as in Library.
So what I'm seeing is that the Develop previews and Library previews are the same, and the Exported versions are similar brightness and noise to the 1:1 Zoom Previews in both Library and Develop. All that seems correct to me. What is still different is that less-than 1:1 zoom images are not entirely accurate since the interpolation in Library and Develop isn't as time-consuming as what happens during an Export and there isn't any Export sharpening being applied in Develop and Library, which is also expected.
Is something else happening for you in LR 5.4?
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