Screen Rotation

Just want you to try two things-

1. Ensure that Zoom is not turned on. Go to Settings-->>Display & Brightness-->>(Scroll all the way down)View and make sure that View is set to Standard and NOT Zoom.
2. Force ReStart your iPhone and here is how -->>Force restart iPhone - Apple Support

You have already confirmed that the Rotation Lock is OFF and that Rotation works normally with other apps as well.

The other thing to do is make sure that your iPhone is updated to iOS 13.

Axel F.

Some apps will not permit Landscape Mode, only Portrait. Conceivably, some wayward link might force a similar restriction: is this happening with all web pages?