When I join my friends Realm I can't interact with with anything, no fall damage, mobs just ingnore me. When I break a block it just disappears.
For my friends I just stand on my spawn point, not moving at all. They are able to hurt me, but after I die I'm stuck on the death screen and have to for close the game.
Found out I have the same problem when joining a friends multiplayer world. The problem seems to be either my internet connection or my router. Because when I used my phone as a hot-spot I worked fine. But I have no idea what exactly it is, how to find out and how to fix it.