Images look bad in Preview.app on El Capitan

Since migrating to El Capitan jpeg and tiff image files look very bad in preview.app with severe posterization issues. It seems that this has been the case since the upgrade to OS-X El Capitan. Never seen the issue in previous versions of OS-X. The jpeg's themselves are fine. They look good in any other application (Adobe Lightroom, PhotoShop, Apple Mail, Word, Pages and Apple's own Photos app). Even Cover Flow in Finder at maximum size shows the jpeg's just fine. Once you double-click them however, the problem starts as Preview shows them with the quality issues mentioned. It especially seems to have a problem with darker tones / shadows ...

It doesn't make any difference whether I save the images with AdobeRGB or sRGB color profiles. Even jpeg's at the highest quality have the same issues. Same goes for tiff's. It also doesn't matter whether the files have been processed or not. Straight out of the camera raw files saved as jpeg show the same issues. It isn't related to the software that creates the jpeg's (Adobe Lightroom in this case). Jpeg's created years ago, that looked just fine in Preview until a few months ago also open up with the same issues.

Is Preview.app over-compressing the files for performance sake or is there something else going on?

I am using a Mac Pro (Late 2013), with plenty of memory and cpu's and two D700 GPU's. I am viewing the images on a calibrated Eizo CG318-4K display. The images are coming from a professional camera.

Below a sample of how an image looks in Finder's Cover Flow (right) vs when opened in Preview (left).

Cheers,

Johan