EDITED!
It appears my last report was incorrect.
Same problem I described happened after I reloaded the game, this time I was just standing on regular ground, but again I got teleported to default world spawn, even tho I have set spawn. I also haven't figured out what killed me, as soon as the game loads I am already on a death screen.

Please help this is making the world unplayable.

Old report:
"What happened was, I saved and quit the game, while my character was standing on top of a leaf block platform high in the sky.
It seems as soon as I loaded the game I was already dead. I also noticed I was in a different place to where I last saved the game (the platform is on a mushroom island and the death cam was on regular grass). I would also note I am wearing Feather Falling IV enchanted boots which didn't seem to be calculated.

I went back , under the leaf block platform, in search of my items, with no luck on finding them.
Then I went to the few places there is regular grass around the island and found all my items in the default spawn position of the map (which is the default "Mushroom Island" option you can choose in the seeds while creating a new map).

So there is a big problem, it seems the game teleported me back to default spawn when I saved the game on leaf blocks but kept me in the same height. Luckily I haven't lost any items, but they could've despawned if I took too long.

Screenshots added to show approximate coordinates and locations I was before and after the death."

Hope this description helps solving this problem and I apologize if it's a duplicate, but I couldn't find.