In User Sync V2 Tool, which is the only available option for user synchronization in DocuWare Cloud, when synchronizing the users with Azure Cloud, you can only use the UNP and the EmailPrefix for usernames in DocuWare. Those two attributes contain the same data and are not at all user-friendly options for usernames to work with. When you adjust the usernames manually to get usable usernames in DocuWare, the next time the Sync job runs all the usernames are being overwritten. Overwriting the usernames practically does not allow using anything other for username in DocuWare apart from the e-mail prefix.
Please, think of a way to use the Azure AD users in DocuWare with user-friendly names or at least, think of a way to configure the User Sync V2 Tool not to update the usernames of already created users.