I recently ran into this problem. I got an OLED 4k TV. Found a random HDMI cable, plugged it into a 2070, and I had a weekend of G-sync compatible glory.
Later on I connected up my PS4 (1080p only) and also tried my PS2 with HDMI adapter, although that didn't work. Didn't think more of it. Later again I went back to my 2070 PC, and had all sorts of display problems, eventually losing signal altogether. Tried different ports on the TV. Tried my 1070 system also, same problem. Why was it working before, but not now? PS4 was still working fine.
Turned out, I had swapped cables while I was plugging in the other devices. Of the 3 HDMI cables I had on hand, only one of them would work at 4k. It was never a problem before because I didn't have a 4k display. The other cables are still fine at 1080p.
Since the above happened, I bought two different certified cables from Amazon, but I haven't tried them yet. They were the cheapest ones I could find. Thought it a good idea to have some more on hand regardless.