This is by product design. The sender of the document is the name of the person which has been configured in the field action called "Initiator" and not the workflow initiator.
Detail can be found on the help file:
The name or email address of the authorizing user who receives the request from Nintex to authorize the Adobe Sign account.
The authorization request can be sent to an email address that is outside your tenancy. An external email service configured by Nintex is used to send external emails.