This is technically a Dupe of MC-3, but since that issue is RESOLVED without the actual bug being so, I guess I'll reignite this issue here.

To summarise:
When traveling to or from the nether the player, in general, gets rotated 180 or 90 degrees. (this is a feature/intentional and not the here mentioned bug!)
This is intentional, but a bug occurs in the latest version (18w07c) which causes the player to not get rotated in one specific case.
This bug or bugs related to this causing simular issues have been in the game for a long time and the specifics of when it occurs and what rotation is or isn't applied may change as well.
But for now it seems to cause the player to rotate 0 degrees every time they travel from the overworld to the nether through a set of portals of which both portals are along the z-axis.
This is a bug, because it's inconsistent as it only occurs in this very specific case.

For some more details. When this bug occurs (traveling to the nether in the specific case) the player may also get displaced 3 blocks in the positive z direction in the nether when they enter the dimension.
So instead of entering the nether inside the portal you entered, you get transported 3 blocks south of that portal.
I myself don't understand why this it happening, but it might as well be a side effect of the main bug here or maybe (very maybe) a whole new bug alltogether.

I tested many different cases with this bug, earlier reported for 1.11.x and 1.12.x, inside 18w07c.
The method I used was as follows:
One portal in the overworld and one in the nether. Take note of the axis each portal is facing and then go through the portal facing south (0 degrees), then back again facing south and then west etc. Then rotate one of the portals and repeat until all four combinations of portal orientations are checked.
I took note of the rotation that was applied to the player after traveling through the portal under those specific circumstances.
I noticed that all outcomes where consistent but one scenario. That's the one described above.