I joined CubeCraft on the New 1.13 Update, and as soon as I spawned in I moved forwards and all of my UI (Pause and Chat buttons, hotbar, movement buttons, sneak button, jump button, etc) disappeared, as well as any other players. All I could see was the blocks, no entities or UI. Happened to me previously on InPvP (Mineville City) also. I am on a mobile Android device.