For a while now they've made the default user account name by truncating the first 5 letters of your email (so if your email starts with 'robert' your username will be 'rober'), and to stop this you need to do a weird workaround where you dodge some UI tricks that get you to login with your Microsoft account so you can create a local user account with a name of your choice.

I believe when I setup my recent machine I had to be disconnected from the internet to be able to complete setup without a Microsoft account.