Screen crossing with white line

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

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