Affects Version/s: 1.9.0
Fix Version/s: None
Platform:Windows 10 - PC
This appears to be fixed in 1.10. Though seeing as I haven't found the change log with this fix mentioned, I'll leave it up temporarily.
From the changelog:
- Player position is now accurately saved when exiting and rejoining a world
When signing out of a game at world height (standing on a block at y= 255, player height = 256), signing back in to the game results in a variety of unpredictable behaviors.
The most frequent possibility is that the player will appear somewhere below the block they signed out on, slightly offset in x or z coordinate value (this is not what is described in https://bugs.mojang.com/browse/MCPE-38374).
The next most common result is respawning in at y = -40 (below bedrock) and instantly dying if in survival.
The least common behavior is actually respawning where you signed off, which would be the expected result.
Steps to Reproduce:
1) Create a new creative world.
2) Place a block at y=255 and stand in the middle of it.
3) Leave the game and come back.
4) Notice the variety of strange results.
5) For more fun, fill a 16x16 area around the player's perch down to y=2 with solid blocks. Move around and retry. Certain positions seemed more likely to result in strange behavior than others.
Moderately severe by my estimate. While it's unlikely that most players build to world height, those that do can encounter severe progress loss. It also indicates a slightly unpredictable implementation, which could indicate further issues not yet discovered.
Unlisted Video Example (7:27): https://youtu.be/r8Ups0My1uw
The end of the video contains two below bedrock spawns, and they can occur even with air blocks below the block(s) at 255.