Tim Horton's allows reward card in Apple Wallet, but not Scan And Pay (which gives rewards and pays for order at same time). I wonder if the addition of the payment creates a dynamic QR code controlled by the app, and therefore not allowed in Apple Wallet for some reason.
EDIT - I think this is the case. When I open and close my Scan and Pay QR code it is changing each time whereas my Reward only card (in Apple Wallet) is static. Must be due to security.