When the onboardee record is opened at the MPH page, their email address is displayed under the username field and not inside the username field.
"Image/data in this KBA is from SAP internal systems, sample data, or demo systems. Any resemblance to real data is purely coincidental."
SAP Successfactors Onboarding
This is expected behavior. MPH screen gives the flexibility to update the userName if required or can be entered as is. Based on functionality and requirement one can re-enter the same email in the box or enter a different ID.
An enhancement request has been logged for the username to auto populated with the user ID in Manage Pending Hires if you would like to vote on it click here
username, below box, field, MPH, Onboarding, value, Manage pending recruits, old value , KBA , LOD-SF-OBX-EC , Integration EC - MPH, Hire , Problem