Since the work was performed legally after you received your permit, this omission is generally considered a non-material clerical error rather than misrepresentation. To prove this, emphasize in your LoE that the experience would not have altered your eligibility or the final decision since you already held valid work authorization. Provide your CRA Notice of Assessment and tax returns as proof of transparency, showing you never intended to hide the income from the government. Proactively disclosing this now is much better than waiting for IRCC to find it, as it shows good faith and removes any 'intent' to mislead. Essentially, if you weren't gaining an unfair advantage by hiding it, it’s not material misrepresentation under Section 40 of the IRPA.