Why have the Azure AD account names recently changed the username on Entra ID joined devices.

I have noticed that Entra ID Joined devices have changed the username of the “AzureAD” username to be the user FirstLastName. Previously the users local Windows account name was their username but now it is their first and last name combined together without spaces. As an example:

Entra ID upn: [email protected]

First name: Test

Last name: User

Original Windows username: AzureAD\Test

New Windows username: AzureAD\TestUser

This has caused problems as we use DUO and that relies on the username to authenticate. Now all of the usernames used to log in have changed and I cannot find a reason why that is.