I am having the exact same issue.
I'm using a 980GTX with 50' HDMI 2.0 on my Samsung Q8FN.
Every time I turn UHD on for HDMI 1 (my PC's input source) I lose signal and the spaz on my other monitor begins.
My PS4 Pro works with UHD on, so I'm assuming it's a software/hardware issue in Windows...