Make sure you have a track in your Play Queue, then drag and drop a track from the Library into the Play Queue. Notice how the track will be appended below the existing one.
Then drag and drop a file from Finder into the Play Queue. Notice how it replace off of the existing tracks!

To me that seems like a bug!
Cause I expect similar experience wether I drag files/tracks from Library or Finder...
Edit: using revision 689