Your "copy last updated by" field ; have you got a user-lookup associated with that in the GUI?
Theres no real reason why (for a custom field) a simple database field would not be displayed in the GUI, so my only thought would be that if the custom database value was a piece of text (eg. "Morton, David") and you had associated a user-lookup with the field in the GUI, then that would fail - since the lookup would be expecting the database value to be the numeric 5000123 id not the text.