I was using my nether portals (on my server) to travel and noticed that in one case, after teleporting, I was not turned to the right direction. When i enter my house portal facing east or west, in the nether I'm facing the opposite (see images 1 and 2). Same for north and south.
But when I go to my farm portal, I enter the portal and I'm turned in the right direction (see images 3 and 4).

I made some portals in a single player game, with a z axis portal, but didn't manage to reproduce this. Maybe multiplayer is the problem ?