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?

Employee Central - Concurrent Employment

Import Concurrent Employment , CE, Concurrent Employment, username, userID , KBA , LOD-SF-EC-CGA , Global Assignment & Concurrent Employment , LOD-SF-EC-EDP , Import Employee Data (EC Core only) , Problem