I am sorry to say it but it is most likely charging port problem.
Not in every case of course but it happens.
My Galaxy S8 Plus died because of faulty port.
You could use Tasker as I do.
Set up a profile to trigger immersive mode when you enter the applications and leave the mode when you leave the application.
If you think about original firmware, highly unlikely expect using Odin.
I'm not positive but have you tried booting into recovery and wiping data? I don't have a rootable S9 plus so I don't have experience directly.
Thank you for your answer, I did. Adding to the post that I formatted data as well!...
OK my twrps for s8+ are based on 3.2.3-2 (official data mount fixed).
So you use the modded twrp from noxx?
If yes you have to reflash kernel and root after flashing one of mine twrp.
It's a known bug when you switch from modded twrp to o...