Why I edit user name but it doesn't sync to other devices?

You can edit your username at any time. Still, to avoid changing your username in such a way that difficult to recognize who you are, the username change will not be actively pushed to other people. The old username will always be displayed if they do not click on your contact card. Of course, there are also exceptions that no matter how you change your username:

• The other people have already added you as a contact, then what your user name was when he added it would remain unless he deletes the contact to sync your latest user name;
• The other people had given you an edited name, what your user name was when he edited it would remain unless he deletes the contact to sync your latest user name;
• When you use some bots, even if you have changed your username, the username that you were authorized with will still be displayed in the bot, unless you delete the authorization and then re-authorize it. 

Resources:

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