This has been reported for the PC version but I can't find it reported for MCPE.

I have made Overworld portals quite a distance from each other but when I go into the nether, I always come out at the same original portal in the nether. This can't be right since it doesn't work that way in the PC version.

Makes it very difficult to move in the nether, especially when you find yourself on an island surrounded by lava. Takes a lot of fun out of the game.