If you remove the Username merge field from the default New User Registration email of the User Login part, you are not able to add it back as it is missing from the merge field option.
We are currently investigating this issue.
1. Create a new user login part 2. Click the green "New User Registration Email" button 3. Select the "View" tab in the editor 4. Click the button labeled "HTML Code" 5. Copy the code that you see here. 6. Click Cancel. 7. Go back to your original user login part 8. Click the green "New User Registration Email" button 9. Select the "View" tab in the editor 10. Click the button labeled "HTML Code" 11. Paste the code that you copied from the other part there. 12. Click Update to close the window 13. Edit the rest of the New User Registration email as needed.
Steps to Duplicate
Remove the Username merge field from the New User Registration email
If you try to add it back using the merge field dialog box, it is not listed as a selection