Ahhh -- you were adding a new book, not copying from the library?
I just tried doing that, and it worked for me.
This operation will use the preference settings for adding books. Any chance you have ticked 'read metadata only from file name' and that your regexp is getting the two reversed?
My settings are vanilla: no boxes checked on the add preferences tab, with the standard regexp. I added an epub that had metadata in it, and the metadata was right. I added a text file and got the text file name as title with author unknown.