Photos Get Marked 'Edited' When They Weren't

I've been very happy since I switched from LrC to Lr - my life is better!

But one weird bug I am seeing is the 'Edited' flag on the bottom right of a photo.

It seems that sometimes (though not always) when I have a folder of files on one drive that I then copy to another (like for instance from a portable drive to my desktop HD), Lr marks ALL the photos as 'Edited' even if only a small fraction of them actually were.

Is anyone else seeing this?  Its an important one for me because I like going back through my folders and sorting by photos that have been edited.

Thanks,

David