Even while the world loading screen (possible only the loading resources part) is still up the world is loaded and things can still affect you. If you were underwater of in some other danger because of mobs, fire, water,  ect… you might die before you even can do anything.

Steps to reproduce. 
1. Open a new world

2. Go underwater

3. Save and quite the world

4. Rejoin the world

5. You amount of bubbles will be less than it was when you left the world, and if the world loading took to long (especially on the loading resources screen) then you may drown.

Expected result

The world does not change until the loading process is entirely done and the loading world screen disappears. If that is to much to ask then at least add a damage immunity until the world has loaded like it is on Java.