You have run afoul of some of the gotchas of the way Groups are implemented in SketchUp. Beneath the skin, Groups are actually just a special case of Components and share almost all of their underpinnings. Because of this sharing, SketchUp implements "lazy" uniqueness on Groups. That is, if you make copies of a Group, SketchUp handles them exactly like ComponentInstances until you alter one in a way that necessitates breaking it off as its own unique Entity. Your "stone/plaster wall opening" is actually a Group, not a Component and your mirrored copy did not force SketchUp to make the two objects unique. This only happens when you edit one or the copies.
What seems like a bug to me is that until you force uniqueness, the EntityInfo window displays "Component (2 in model)" when the underlying definition is actually marked as a Group and the supposed Component does not appear in the Components window (I confirmed the marking by probing via the Ruby Console)! Note that being a Group or Component has nothing to do with the name of the definition. The sash in your sample file is a Component with the name Group1231!