Really looking forward to the bug fix in rush as well.
Unity2018.1.0 + vuforia7.1.34 + iPhoneX(11.3.1)
Unity2018.1.0 + vuforia7.1.34 + iPad Pro(11.3.1)
Unity2018.1.1 + vuforia7.1.31 + iPhoneX(11.3.1) <= also reproduce the no background video bug
Unity2018.1.1 + vuforia7.1.34 + iPhoneX(11.3.1)
All of the above can reproduce the flipping orientation bug with the rate 100%
But for unity developer, I may found a temporary solution.
It seems the ground plane cause the orientation of the child object to flip , but the position remain the same.
I try to make the AR object not the child of the ground plane object and attach a script that let AR object follow the ground plane object with fixed offset.
(transform.position = groundPlane.position + offset)
The result turned out to be acceptable.
My demo video: https://youtu.be/MEXEgUTFScw (Flipping happened at 0:35)
However, it's just a quick finding, I am not sure whether this solution could cause new bugs.
Any discussion is welcome before the bug fix is released.