It seems I have a similar issue with the filter env amount - to get a true zero, I have to set the knob a fraction to the left of the detent. Not an issue normally, but with our ears being so sensitive to pitch fluctuations, it does become noticeable when using the filter as sine oscillator.
However, a bigger issue for this particular technique seems to be the keyboard tracking, for which I cannot find a setting at which the pitch tracks the keyboard (even roughly) for more than an octave.
Has anyone had more success with this?