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.