You can 100% change the passport back to your Canadian passport yourself online for the return leg. There are 2 ways: first is to update the passenger information page after you click check-in on the app but before you actually start the check-in process, or, as someone else mentioned above, hit the Back button during the check-in process when it asks for the ETA details and replace the passport info with your Canadian passport. Not exactly intuitive but when once you do it it’s easy enough on later trips.