Usually Develop-Fit view is crispier than Library or Export, when there has been enough noise-reduction added to an image to make it seem smooth, and people complain that sharpening is not being applied; however, in this situation through extreme noise-reduction settings, an artificial coarse texture has replaced the fine-grained noise which seems to manifest differently in Develop Fit view, and the complaint is that Library has no NR applied.
In more detail, there are two issues, here, both of the user's doing, not LR's:
1) A user-expectation that the Develop non-1:1 zoom is accurate regarding Detail settings. It is not. For years there has been an exclamation-point warning on the Detail settings section that says to view things only at 1:1. If a user doesn't see or ignores this warning then their expectation is wrong, as in this situation.
2) The Luminance Noise-Reduction Detail slider has been increased from the default of 50 to the maximum of 100. This has the effect of converting the fine-grain noise into a course angular texture, which is easily visible at reduced sizes except in the inaccurate Develop Fit view.
In the following side-by-side at 1:1 zoom, the lefthand panel is as per the user settings with the Lum-NR-Detail slider set to 100, while the righthand side has the Lum-NR-Detail slider set at the default of 50, with all other user settings remaining the same:
I would summarize what is occurring not as a bug but a different response of the inaccurate Develop-Fit rendering to a different coarseness of noise that is caused by an extreme setting of the Lum-NR-Detail slider. Since there is no resizing and no sharpening in the Export settings, I don't think the 1/3 bug has anything to do with anything.
It would be nice for the original poster (OP) to describe exactly what is wrong, after knowing that the Develop view is in accurate if not viewed at 1:1.