The bug

When working with multiple singleplayer worlds (not tested on multiplayer), the player will spawn in at the wrong location, holding the item they had in their hand from the previous world, before being teleported to their correct spawn location and that item then disappears from their hand.

This issue has the occasional effect of playing the water enter/leaving sounds depending on the seed you're using and the environment. You can replicate the player position and item holding issue flawlessly, but the water sound effect one is a bit harder to repeat consistently.

How to reproduce

1. Make sure Minecraft is windowed (this will be easier to replicate.)
2. Create a new single-player creative world
3. Launch the world
4. Place any item from the creative menu in your hand
5. 'Save and Quit to Title'
6. Create another new creative world
7. While it's generating, tab out to another window, or click on the desktop – anything to remove the window focus from Minecraft
8. After the world has generated, the "Game Menu" will appear. Notice you're holding the item from the previous world you were playing. Notice also your surroundings may be different from those you spawn into.
9. Click 'Back to Game'
10. Observe that the item you appear to have have teleported to your correct spawning location (if surroundings were different) and the item you were holding is now gone

Now we will repeat this on the other world, demonstrating the issue is consistent

1. Place a different item in your player's hand
2. 'Save and Quit to Title'
3. Launch the first world you created
4. Tab out, or click the desktop, to remove the window focus from Minecraft
5. After the world has generated and the "Game Menu" appears, notice you're holding the item from the previous game and that your location surroundings will be different to the ones you're about to spawn back into

Notes

As I have explored this bug more and more, I've managed to replicate it consistently with the above steps. I have uploaded one final video demonstrating the entire bug, including items in hand. Please click the YouTube link for the best representation of this bug: https://www.youtube.com/watch?v=mRfegVLshnk