So Nurtures have Entry and Exit. Exit is based on a Goal, such as X form fill outs, MQL, etc... Exit can also be based on unsubscribed.
It doesn't really matter what your form fill out is, just matters that you choose a Goal.
So for your issue, I would say what you did - if the lead is in Engagemetn AND fills out form (trigger), then this is your Goal and mark the person as success in the Engagement only.
If they happened to have filled it out in the past, so what? Maybe they shouldn't be in this nurture then.