I have been wondering the same thing and have come to the conclusion that this is normal. Partly because that's the way it works with both my Werkstatt's...
Here's my theory as to why it works this way:
Turning the 'Sustain' switch off makes a very short pulse driving the envelope generator. Another way of saying this is that the 'window' changes from however long you hold the key down to a non adjustable short window. At the end of that pulse, the release kicks in (the Werkstatt refers to it as 'decay' even though Moog is the originator of the ADSR terminology...). Unless your attack is short, there's just not enough time for the envelope to open up the VCA very far. It would be nice if the attack section were allowed to develop and trigger the release when it came to full blossom but I don't think that's how the circuitry has been designed.
Perhaps someone will design a mod that switches from current behaviour to a three way switch that allows sustain/short pulse to trigger the attack/full attack blossoming(trigger at the end of the attack cycle).