1. Make a document with linked files preferred.
2. Place a PDF (or anything else).
3. Make this PDF embedded in Resource Manager.
4. Edit document (embedded PDF) and make some changes to it.
5. Close Edit document window.
6. Now take no other action than "Undo". Should be now undoing embedding the file.
7. Visit again Resource Manager, file is linked again, but now the placed file still looks like the changed one and is not reverted to the "look" of the linked file.
8. Save this file and the preview icon of the .afpub shows the changed file / look / layout.
Closing and opening the document again, shows the correct linked file. This just means, that undoing embedding is not instantly redrawing the "appearance". So no bug at all, just a minor flaw. Just FYI.