So I've tried both using the Native ARKit plugin with Swift and XCode, and also Unity's ARKit plugin that exports to XCode and have noticed the same issue
In either case, although it is very quick and easy to do plane detection and then do object placement on the plane, my users have pointed out..... if we want to place a (large) object on the room's floor after it is detected as a plane, it always seems as if the object and plane is floating on top of the floor rather than being on the floor itself.
Just before I posted this I downloaded a couple of tutorials on github to see if it was perhaps my code doing something wrong, but in the samples I've just tried on github, I noticed the same thing.... if I let the phone (iphone x) scan the floor until it finds a plane, the plane itself seems to be above the floor rather than actually on it
Any advice on this?