Using Avatar SDK 3.5.2-beta.2.
If your avatar was exported from Blender using the FBX export Transform setting, "Apply Scalings" as "All Local" any child of the head(or head itself) that should be affected by VRC Head Chop to show in first person will not show up in the first person view.
Re-exporting the Avatar from blender with the "Apply Scalings" as "FBX All" then in unity resetting the pose on the fbx rig avatar definition corrects the issue. This fix breaks all animations that modify the position of a transform, resulting in you having to re do all those animations.
If this issue cant be resolved it might be a good idea to update the docs for VRC Head Chop that the scaling "All Local" is not supported for it.