Hi there!
It would require some discussion as a potential new feature if and/or how we should populate list for Component reference variables in Details view - currently Component reference variables cannot be assigned at design time (as you have discovered here). So technically, current bug would actually be that we are exposing those reference variables for editing in Details view for Component defaults in Blueprint Class editor (because we don’t currently have a facility for assigning them at design time - i.e. they are not “assets” per se).
Note that if you create Component reference variables for an Actor-based Blueprint Class, those do not appear in Details view for editing class defaults in Blueprint editor. I believe that is actually current intended behavior.
Also note that we do currently handle Actor reference variables as a special case - in Blueprint editor you cannot explicitly edit them, however in Level Editor’s Details view you are able to assign those variables at design time for a particular instance of Actor-based Blueprint Class to any Actor instance in current level. Unfortunately we also currently expose Component reference variables in that context as well, for which drop-downs will also be empty. Whether or not they should be exposed there for per-instance editing would also require some discussion as a potential new feature request - for example, UI for that might be a bit tricky.
In 4.8 you’ll need to continue to assign those references through Construction Script (as that is only currently-supported method). In meantime we can consider adding an ability to perhaps be able to “predetermine” those values somehow at design time (through tagging or otherwise) so that when Blueprint Class is instantiated those assignments could be set up automatically. I’ve added a new issue for tracking that as a request (reference UEBP-57).
Thanks for feedback/report!