I've had my OnePlus 6 for about a year and 5 months. This problem with my screen flickering started at around the 1 year...

The only answer is replacing the screen im afraid. When the screen does this, it's because the LCD is fcked. I replaced my screen and it works just one now. No black screen or anything else wrong. It's expensive yes, but if you can get a warranty on the parts replaced I'd do it. Then you'll have a phone which works 4-5 years in the future.. I'd say it's worth it but y'all might have a different opinion.