This was never issue for me before. Started 2 days ago.
Since I had seen others while searching for a solution to this issue and since It was driving me nuts too I decided to post my solution.
Photos, whether RAW, Lightroom edited RAWs or Raws edited in Photoshop and saved as TIFFs the Images were displaying in the Development module looked very different (typically darker and contrastier) from library module's version.
Steps used to try and fix. NONE of these worked in my case:
1- Verified my monitor's color profile is not corrupt
2- Verified my video card driver is current
3- Synchronized the image folder to make sure there weren't any edits not being reflected
ACTION THAT DID WORK:
I noticed that if I zoom to 100% in the Development module then move the zoom slider back to "fit" this difference dissapeared.
So I discarded all smart and 1:1 previews and rebuilt them. This seems to have resolved the disparity between the 2 modules.
I now have peace again in my photographic world.
Could you explain how to discard/delete all previews (Smart and 1:1)? Under the histogram, mine shows 'Original Photo' and when I click on it, the message asking to build a smart preview of this photo appears. So I don't have any smart previews to discard.
BTW, if I use the zoom commands in the Library module like you describe (first zoom into 100% 1:1 then back to FIT) my Library Loupe view is cleaned up and looks identical to the enhanced Develop module view. This is a bit of a hassle because it takes a lot of time to go through 100s of photos to fix their Library view image.