This is definitely strange. The only information coming from the headset (to the app/Unity engine) is the the view geometry and the recommended resolution, which basically means two configuration options can potentially impact that:
- Parallel projection,
- FOV setting,
and the impact is indirect - i.e. the headset is not directly involved in the 3D corruption, but rather some bug in the engine (or the model data) is exposed by the different view geometry and this clearly affects only the hand model (and not the controller).
Smart Smoothing or FFR are basically done in “post-processing” and cannot corrupt specifically only one object’s geometry (and not affect the other objects in the scene).
Now for what concerns the former two, I could imagine problems with Parallel projection (or, to be precise, with the lack of it), eventually with FOV too big, or recommended render target res too big, but neither seems to be happening in your case (Small FOV with PP on should be close enough to Vive geometry).
Just to make sure, you restarted SteamVR every time you switched the FOV, or parallel projection mode?