When you create and enter a portal in the Nether, and a new portal in the Overworld is created on top of a tree, entering it will send you below the portal in the Overworld but not inside of it.
This relates to MCPE-28765 but isn't really the same. Note that the issue is really hard to reproduce because the game randomly chooses a position of a portal to be created, if the exact position is obstructed.
I'm reporting this on behalf of my friend. While I tried to reproduce this separately, I wasn't lucky enough with the RNG.
Steps to reproduce:
Create a new world with seed -725457214.
Create a nether portal at somewhere near the initial spawn. Then enter it.
/tp to 196, 79, 63 in the Nether. You'll teleport into Netherrack.
Break blocks to get some space.
Create a nether portal at 196, 79, 63 - 199, 83, 63, then enter it. Depending on the RNG, a new portal may be created on top of a tree in a swampland.
When that happens, players who enter the portal will show up in water below the tree instead of the portal.
Here's a video showing the issue:
http://starlight.sakage.jp/~nimms/mp4/Minecraft_201908140541.mp4
There is no bug here. If you watch the video closely you'll see that the player does not spawn below the portal in the water. S/he spawns at the expected position in the portal (1590, 69, 492), walks forward across the treetop 2 blocks to (1588, 69, 492), then continues moving forward while falling down out of the tree, finally landing in the water near the tree at (1585, 62, 492). This is just a player walking out of a portal and off of a treetop while the purple portal haze overlays the screen.