Why can I still log in to the app, even selecting a different country code from that at the time of registration?

When you signed up the mailbox, the country code is used to select which country to sign up in. The real account of the mailbox is used without the country code. So next time, you can log in with another country code under the same account.

However, if you sign up with your phone number, the real account would be 86-1388*******. So when you log in with your phone number, it will prompt to log in and report an error if you switch the country code.