Why does shadow appear in my iPhone 15 Pro Photos?

I’m unsure what you’re referring to as a shadow, here.

Monitors tend to be challenging subjects for photos.

There are mundane shadows near the desktop, below the monitor.

What looks like a wall crack or gouge in the lower left corner of the image, too.

Monitors can have blotchy backlights and reflections of other light sources. Your display certainly shows various reflections from ambient light sources elsewhere within the room. What looks like a backlit person, among. Purchasing a monitor with a display coating can sometimes help reduce this, such as Apple’s nano-texture display coatings.

The image also shows ghosting of the brighter sources within the monitor display too, slightly above and to the right of the sources. Ghosting is a form of lens flare, and is caused by reflections within the stack of lenses used in the camera.

Another photographic artifact that can arise within a lens stack is Newton’s rings. Your image doesn't show that artifact, though. Auroras are a source that can.

That’s your reflection on the screen you’re photographing. It looks like you’re standing in a doorway with a brightly lit room behind you, when you took the photos. Reflections happen and they can also be solved. Just follow the Law of Reflections and observe where the angle of incidence is.