I am on a Mac Mini (2018) with Big Sur 11.2.3 and LrC 10.2. I often use the LrC book module for printing photobooks via Blurb paying a lot of attention to the quality and layout of my images. Since Big Sur I discovered that there is a major and bad colour shift in the preview of the image when using the command Zoom Photo To Fill Cell in the book module, also using the zoom function in the cell causes the same issue -> image is getting a kind of desaturated result! I read discussions in other posts of comparable problems with zooming in the print module and in the library module, and those problems still seem to exist. I found out that those problems don't occur on my MBP (Retina, 15-inch, Early 2013) with Catalina 10.15.7 and LrC 10.2. So it seems that LrC doesn't work fine together with Big Sur in this respect. I hope that this problem will not affect the quality of my images while sending a book to Blurb. I may expect that Adobe will solve this problem now within due time.
left picture is not zoomed and is correct, right picture used command Zoom Photo To Fill Cell, in this case it was zoomed up until 3% If you use the zoom function in the black bar on top of the cell manually up until 3% - same result, clear differences in colour,
another strange thing in the book module is the small black lines which occur in the preview all the time on one or the other side of the image, see next screenshot, maybe this is connected to the colour shift issue!?
furthermore the colour shift issue also occurs in the print module. If you uncheck Lock to Photo Aspect Ratio and changing the size of the image which deviates from the aspect ratio of the photo, the same thing happens, see next screenshot. You can adjust that however by using the command Match Photo Aspect Ratio, than it turns back to the right colours,
before Big Sur those things did not happen in LrC - looking forward to your reply
Others in that forum have reported the same symptoms. It's pretty clearly related to bugs with Library zoom and Print zoom, but Adobe employee Rikk Flohr requested that it be filed as a separate bug report.
Please continue all discussion in the bug report thread, where Adobe and others can easily find it.