First I would like to thank Aya for providing the clear documentation of the issue. I noticed the exact same problem after installing the same wall mount for my TV.
When I rotate the the joint at the wall mount the TV stays horizontal from left to right. The same happens when I rotate the middle joint. The TV stays in it's horizontal position.
But when I turn the TV from left to right the TV doesn't stay horizontal and does exactly what the pictures show. This is quite annoying.
I don't really see how locking the rotation stops this issue. On the contrary, it will make things worse because I can no longer correct it to a horizontal position the when the joint at the TV as not straight. I tried during installation.
The problem seem to stem from the metal (black) stub, to which the TV mount is connected, being tilted slightly upwards. I cannot find a way to adjust this.
I would have expected the black metal stub to be horizontal as well. Then this issue would not exist in my view. However the position of the stub is kept perfectly the same by the double hinged arm mechanism and I see no way to adjust it.
The LX arm moves in several locations, so portrait/landscape acts to level the screen when other rotation points are turned away from the center.
I don't really understand how this should work. There are 3 joins that can turn in the horizontal plane. 2 of them keep the TV in the horizontal plane when rotating. The third one actually rotates the TV over a tilted plane?
I really like the quality of the product, I could recommend it to anyone except for this issue.