That’s mentioned on the Direct VR page in the vorpX ingame menu. Understand it as a new mean trick of GLaDOS to confuse you. :)
Seriously: while in other games something more elegant is possible, for Portal 2 Direct VR unfortunately needs to blindly overwrite the camera rotation with its own value, otherwise it jitters heavily. In other words: Direct VR does not take any rotation reset done by the game into account, which happens when you go through a portal. Unfortunately all attempts to reliably circumvent that or alternatively detect a reset by the game and react to it failed so far.
I will take another stab at that for sure since Portal 2 is such a great game in VR, but I can’t promise whether a solution will be possible.
Does anyone know if there is a change to this? I only ask as this comment is four years old, was kind of hoping for a resolution!