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

Description:

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.

Severity:

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.