4K external monitor gets my MBP monitor stuck with 1920x1200
 

I have 2 4K monitors connected.
They now and then flicker sif I use DisplayLink...
Additionally, my MBP monitor is stuck with max 1920x1200: don't bother to point me to the support page, that doesn't work, in facts I don't get a list of resolutions, rather the Scaled text icons, and the "More Space" one is 1920x1200.

Mind this doesn't happen if I use HDMI adapters and cables, the 4K monitors and laptop monitor work just fine and as expected, full resolutions.