You made a mistake, so in order to show that you didn’t mean to mislead them you have to write down a letter to explain what happened.Okay - so you suggest to send the form and letter of explanation even if they haven't asked for it this time.
As for the form, usually ircc asks for it when somebody misses a trip, to save yourself time, I suggest you do it, but review it carefully.
No harm in explaining that it was an honest mistake and you just realized it happened
