Hello,
We encountered a problem that is if we move a AGSPictureMarkerSymbol near the screen border, they are getting slightly bigger. You'll see in the attached project two images that aren't touching when placed in the center but slightly overlapping on screen edge. Also if you zoom on the orange marker you can clearly see a differance in it's size.
I think this is the expected behavior for a 3D scene. It is different from a 2D Web Mercator map.
When you pan and zoom on the scene view, the perspective would change. Think as if you are looking at them from a helicopter, their relative position will change as you look at them from different angles.