Then they do absolutely crazy weird things!
I recently got a new laptop. My account is `adavis@<domain>.com`, my user name on my old laptop using that account is `adavis`.
What did Windows 11 do when I create my user on laptop. Oh it makes my user name `adavi`, yes it truncated my username.
After scouring the internet, trying a few different things to rename my account to no avail, nothing worked! Until I found a command to bring up an account management window that looked dated to the win 2k era ish (and can't be found via any settings window). It allowed me to create a local account with the name `adavis`. I then logged into it, deleted my `adavi` account then was able to associate my new local account with my Microsoft account.