Older iPhones and newer iPhones have a difference in the way that they set up AR in the real world.
Older phones do not have Lidar capabilities to place an object. This means that instead, it has you slowy move your phone around to scan the environment. What is happening is that the phone is using machine learning to pick up on planes and surfaces to place your object.
Newer phones use Lidar and are able to make quick and precise points to place your AR in the environment. This also means that if you move behind an object, your AR will be hidden by that object through occlusion.