Follow Object Problem (Out of sync)

Cinemachine's SmartUpdate works by tracking when the target's transform gets modified. If it is modified only in FixedUpdate, then the vcams tracking it will update in FixedUpdate.

The question is: how does the target that your proxy is tracking modify its transform? If it gets modified only in FixedUpdate, then your proxy should do the same. If, on the other hand, your actual target has Interpolation on and is moved in a compatible manner, the physics module will patch things so that the transform is modified exclusively in Update, making it compatible with other behaviours that work in Update. If this is the case, then your proxy should update in Update. This is the preferred way to work in Unity, and it will give the smoothest results.

If your target gets its transform modified in both FixedUpdate and Update, then you're out of luck - it's unlikely that you will be able to sync anything to it satisfactorily. You should fix it if that's the case.

A good way to find out how your actual target (not the proxy) is updating is to use it as a direct target for a vcam. Then, put the CMBrain in SmartUpdate mode, play the scene, make that vcam live, and look at its inspector. If it says FixedUpdate, then you know that the target is moving exclusively in FixedUpdate, In any other situation it will say LateUpdate.