It's difficult for me to provide a video since the scene view and game view don't show what I'm talking about, this is just on the headset's display.
My terrain trees rotate visually. When I say visually, it just looks like the perspective is being changed on the trees (the base doesn't rotate at all, just the tops). This only occurs with VR headset movement. The trees I've placed as regular objects don't have this rotation bug.
There's no code to show either since I haven't even coded anything yet.
Using SteamVR, Unity, and a Vive.
I did some reading and stumbled over this post:
https://forum.unity3d.com/threads/using-tree-gameobjects-vs-unitys-built-in-system.183224/
It explains that the unity generated trees are displayed as billboards to reduce rendering cost I would assume.
Billboards are generally know to look weird in VR, since their rotation algorithm messes up in some way. You can override it for normal billboards, but I don't think the terrain generated tree is changeable, unless you want to dig very deep.
Hope the post I linked and looking into unity billboards will help you understand.