When I move Sbstance Sampler window to my LG external display, the mesh center moves to the corner and I can't center the item and orbit the object properly. The camera insists rotating around some point off center. There is no problem when working on my Macbook Retina screen.
To avoid this issue set your LG screen as main screen in your settings. Go to the Apple icon on the upper left of your screen then click on "System Preferences" > "Display" Then select the screen and set it as main screen.
It might keep the preferences so if you unplugg the LG screen everything would go to normal and once you plug the LG screen back it will reset automatically the LG screen as main.
Then make sure to open Sampler once you have your screen set to avoid any other issues.