You won't start at where you would spawn until you leave the escape menu

To recreate this, create a new world and while it is loading, switch to a different window to make it open up the escape menu once it loads, you spawn a dozen or two blocks away from the actual spot you would've been at, click back to game and your character teleports quickly to where they were supposed to start, Sometimes you will see your character flying above an ocean! Not too critical though.