I just tried it.
Now it is a little bit better.
After the installation of the beta driver the picture was OK. Perhaps because of the setting of the old driver.
Changing it to another resolution like 1600*900 for testing results in a stretched 1920*1080 picture.
Changing it back to 1920*1080 is goes back to 4k with a stretched 1920*1080 picture :-(
After being one time to 4k it doesn't matter witch resolution is selected. The output is always 4k.
But the manual override woks with this beta.
Using "show all modi" and selecting 1920*1080*32 60Hz show a real 1920*1080 resolution with 60Hz.
Some improvement, but not fixed.