When using the username generator the email address field (within the “plus addressed email” in the “username type” setting) defaults to the one associated with the account. It’s able to be changed to a different email, but this has to be done each time the username generator is opened and used. So, if a user wants to use an email that is not associated with their Bitwarden account, they have to manually retype the email they’d like to use each time.
Saving the email used before (for the plus addressed email option within the username generator), and not defaulting it to the Bitwarden account email address would be a a neat time save but not sure how hard/realistic this would be to implement.