Lr 6 Changed the Order Of Photos Upon Import

I use the format of YYYYMMDD(file number).nef when importing photos. And I have Lr arrange the photos in order of creation (or by filename). This last import, Lr jumbled up the order of the photos in the Library? I have no idea what the new order is, or how to go back to my preferred method.

Oh, I have a Nikon D5000, and my laptop is a MacBook Air (with latest IOS and updates). This particular import issue has only happened today, and yesterday the import was fine.