I have this exact issue. It seems to have appeared since the OS4 Update. Ive opened a ticket through the Feedback feature in the settings.
Changing the refresh rate to 60hz resolves the issue for me.
I got some basic troubleshooting from their end: System Update, App Update, clear App Cache, seeing if that Problem exists in Safe mode, disabling adaptive brightness and of course Hardware inspection.
If that doesnt Help they offered RMA.
I will Look into these steps now.
Though for me it seems to be likely software related because of the Update.
Edit: restarted the phone normally. And its gone!