I was building a very tall structure in my Minecraft Realm (I am the owner). I was building a spiraling staircase and everything was going fine, right up until I got to Y level Y=255. Immediately once I started building at this layer, I started taking damage for an unknown reason. Eventually, I actually ended up dying from this and upon receiving the death report, it said I had died due to "Fall Damage", even though I was standing on solid blocks and definitely had NOT fallen. 

Me and my friend on the realm started testing it and we found that what initiates the 'fall damage' is either placing blocks or mining blocks at or above Y=255.

Also, another bug which is shown in the clips below which I believe may be related to this is the block placing glitch. In Bedrock Edition, it is a very common glitch for a player to place a block only for the block to disappear and return to the players hotbar. This issue is shown in both clips, and has occurred on many other instances as well.

This bug has proven to occur across multiple platforms, and occurs on both realms and single player worlds. 

STEPS TO REPLICATE GLITCH:

-Open a single player or Realm survival world

-build up to Y=255 or above

-begin placing blocks in any direction

-stand on one of the blocks without any blocks underneath it (the bug does not work if there are blocks below the one you are standing on, even at Y=255 or above)

-begin dying to inexplicable fall damage

Link to first clip:

https://www.xbox.com/en-US/play/media/DSLGQJLNN6

Link to second clip:

https://www.xbox.com/en-US/play/media/2V4KVLGXJS