Update: now that more people have started using the Artifact Comment feature in the project other user names are being pre-filled into the "Directed to" field - sometimes just 1, other times 2 users and always members of this single project but in multiple modules - we don't see this behaviour in any other project in the database.
There is however no logic to which names are pre-filled - it always is 1 of the project members if only 1 name is pre-filled but if there are 2 names pre-filled the 2nd name can be different (not always the same 2 names).

Eg. Username99 always appears pre-filled in the "Directed to" field but sometimes accompanied by another user - such as Username99 and Username5 other times with Username99 and Username23 othertimes Username99 and Username68, etc. but all are project members.
 
Is there a setting for Artifact Comments that we somehow triggered to pre-fill with content?