So its reasonable for the old user to login again to sync their account to flynax.
No, it is not reasonable for a paid service.

Wei Hong, to make myself clear, I haven't been warned that this would be a compromise solution from the start, so i accepted to pay for the cross login functionality thinking that it would be a full cross login functionality, not just a patchy way to allow 'easier access' for members to log in.

Right now they are working on fixing this patchy solution... so they were reasonable enough to realize that they can improve the service i paid for.