Screen crossing with white line

If that happens using that app only, it could be by design (contact that app's developers).

If it happens everywhere (on all screen pages & using all apps) then have your iPhone inspected at an Apple Store or AASP.

Regards

GIulio

If the issue is restricted to that app, as already suggested, contact them:--> How to contact an app developer - Apple Support

Regards and best of luck

Giulio