With a problem like that, the first thing I would try is plugging the cable into a different port on the back of the graphics card. If that doesn't work, the next thing I would try would be a different cable (the shortest, highest quality one you can obtain). You might also try HDMI instead of DP.
Morning, I have tried 5 cables, the one that came with the monitor, some I had already and then 2 from the certified cable list for DP 1.4, all have the same issue. The issue only happens on drivers after 20.9.1 so thinking its a driver issue.
Looking at the AMD Community forums seems everyone with the LG 34GK950F has the same issue but differing drivers and cards.
I have tried HDMI and it doesn't do it on that so wondering if its a card/driver/ultrawide issue on DP 1.4 with the drivers. Its a strange one, I have tried all you suggested, only thing stopping me going HDMI is that this panel only supports 85HZ on HDMI and 144HZ on DP 1.4
I tried removing the monitor and putting the 2 x 24" 1080p 144hz monitors on DP and they also do the black screen or change res to a lower on when waking the monitors. Still digging about to try and sort this, happy to stay on 20.9.1 for now as all the games I play work.