I'm guessing you're using the stock firmware from Samsung, if so:
- Are you on a beta? If so, it's probably just a bug that'll be fixed soon. If not, you should probably contact Samsung. Since they're focusing on the S9, this bug might have slipped through their fingers.
- Also, try checking your default apps, or resetting the default configurations of some apps.
- If not, It might be a result of screen ghosting, but that's unlikely since your issue is so specific.