Reproduction steps:
1) Open user's attached "GenericBehaviours.zip" project
2) Notice that 'Implementation' asset (Implementation.dll) in Project window has dropdown with 'BehaviourImplementation' object/class
3) Move 'Implementation' asset to the 'MoveImplementationDllHere' folder
-- Notice that you can't check 'Implementation''s classes (Implementation.dll asset doesn't have dropdown anymore)

Expected result:
We should see what classes does 'Implementation.dll' has

Note: From this points you can move Implementation.dll asset however you want, dropdown wont appear again until you put Implementation.dll into same folder where GenericMonoBehaviours.dll.

Reproduced with:
5.6.0b9, 5.5.1p3, 5.4.4p3, 5.3.7p4