As far as I know currently, affects Xbox One (Encountered on Windows 10 joining an Xbox world in multiplayer)

When entering an end portal, as best as I can understand from my testing, the game thinks that the player has fallen from whatever y level the player is at to whatever y level the platform is at, causing the player to take the equivalent amount of fall damage. I first noticed this from entering an end portal that was placed at y=200 while switched into survival, which caused me to instantly die. I then placed a portal much lower down and noticed how I didn't die that time. After testing this some more, I found that it actually deals the exact same fall damage as a fall of the equivalent distance, as the obsidian platform was at y=49, and a fall from a block at y=70 with no equipment to a block at y=49 left me with exactly one heart, and entering the end portal at y=70 also leaves me with exactly one heart.

To reproduce, place an end portal at a high y coordinate and you should take fall damage