FULL STEPS that developers should take to reproduce the problem:

1) Go to Site Administration -> Grades -> General settings and modify grade_export_userprofilefields to contain middlename field
2) Attempt to export gradebook to CSV in some course Course Administration -> Grades -> select "Export -> Plain text tile" in dropdown box

WHAT IS EXPECTED:

1) middlename field present in preview and in downloaded file

WHAT ACTUALLY HAPPENS:

1) middlename field NOT PRESENT in preview and a file