I'm thinking potentially faulty light sensor on the phones. At this point I think getting the phones looked at might be the best bet.
There was an update released today for the phones, try to install it and see if that makes a difference before getting them looked at.
Please keep us updated.
Never think a flagship phone with such a terrible display like this. It happens just after warranty 1 month.
Both my S9 plus and my S9 have the same problem with the screen. The color is not vivid anymore no matter any brightness level. After the half level the phone will completely black out.
Already took them to Samsung center in local. It's hardware problem. It costs me about $300 for both S9 and S9 plus screen.
It's really suck. Never buy any flagship Samsung phone again.