A workflow rule with an email alert set to use the Default Workflow User sends emails from an unexpected address because the user's My Email Settings return address overrides the email on the User record.
This occurs because the return email address in My Email Settings takes precedence over the email address on the Default Workflow User's profile page.
In Lightning Experience
This email address will override the Default Workflow User's email address on the user page, if it is different.
See Also:
Identify Your Salesforce Org’s Default Workflow User
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