I seem to remember giving a possible answer to this in another thread. Specifically: Daimons and Spirits of Place?
The Aspect is trapped because the Daimon could not re-manifest an Aspect there soon enough to prevent the giants and dwarfs from taking over the site. So it has to remain on earth until another solution presents itself...such as the characters coming by.
You might allow it to be possible for an Aspect to regain Might using vis, even if it doesn't naturally regenerate. So the trapped Aspect consumes the vis of the Rhinesfals to replenish its Might as per RoP:M.
Mark