The most common cause for this is when this field has been updated via an import, the user's name has been used by mistake.
For a relate field it needs to be populate with the id of the user.
So for example if Joe Bloggs' user id is 12345, if importing the text Joe Bloggs, this will create this issue. Importing 12345 will show the field correctly.
The fix for this will be to either reimport, or manually choose the user on the affected record
