After defeating the end dragon and going back into the end dimension. If you dig below ground on the main island and place torches, they disappear the next time you come back into the end dimension. They are just eliminated, like you never placed them. They stay above ground when placed (like near the portal), but not underground. This hasn't been tested on any other island, but it does happen on the main island where you fight the dragon.

Before and After example images attached. I placed torches in the room where you spawn in when you enter The End. I then went back through the portal and then returned and the torches I just placed are gone.