Pressing a right click with a hand (or any other item) on some blocks makes them disappear and randomly places some randnom block (grass block, leaves, grass...). See the GIF
UPDATE: When i joined the same world in the "c" snaphot, all the blocks were in their earlier place (before right clicking them) and 2 messages appeared :"Unable to locate a sign at 587, 64, 319" and "Unable to locate sign at 587, 64, 317"