2850862 - Why Username is same as UserID while creating Concurrent Employment via Import ?

When we add Concurrent Employment via UI, the new username created for secondary assignment is created as the main assignment username-1, username-2 etc. When we import the additional assignment, why username assigned is same as UserID passed in the file?

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SAP SuccessFactors Employee Central

Import Concurrent Employment, CE, Concurrent Employment, username, userID, Import, person-id-external , KBA , LOD-SF-EC-CON , Concurrent Employment , Problem