I have exactly the same issue shown in this video https://www.reddit.com/r/GalaxyS9/comments/ffegry/samsung_s9_screen_backoutflickering_issue/
Specifically the screen goes black. This looks like a common issue for S9/plus phones. For me, this happened after the update. I tried restart/clear cashe/factory Reset but still having the issue. After installing "OLED saver" app (suggested by some users in this forum), it is partially fixed but the screen flickers during video calls. I am very disappointed.
I have been a loyal customer of Samsung phones. This is the first time I am facing this issue.
Interested to know if Samsung has reached out and provided solution, as we have the same experience with Samsung Galaxy S9+ and still awaiting for Samsung to reply with a solution. Very disappointing considering the cost associated with the phone.