This is a new thing. Applies to all newly imported images.
Could it be that there's a secret preset that's doing it?
When it first imports, it shows you the jpeg that is embedded in the raw file. When you go into develop, it renders the raw data directly. After this, the views in Develop and Library should be the same. The darkening is very common if you have a dynamic mode set on your camera. Those modes underexpose the raw and write a flag in the raw file to compensate but only the camera maker's software can read this flag, so third party software will render the raw darker than the embedded jpeg.
Thanks a lot Jao, makes a lot of sense. Who knew of these obscure technical features!
Apologies for the delay in responding.
Take good care,