Further to my last post, I've done some experimenting by moving messages back into the inbox. If the messages are in the Inbox, then they are attributed to the correct author. If they are moved to sub-folders of My Folders, then they are not correctly attributed.
It shouldn't matter what folder they are in, the message should correctly visually indicate who sent it. This worked prior to a couple of months before my original post.
There's an Ovi Suite update out; have to see if that's fixed it.