I'm also dubious about this claim. I'm sure it was said that all gifts/ loans to Diana PoW from the Queen or via official avenues were promptly returned to the Royal household on her passing. Would I be presumptuous in saying that gifts to Diana from top members of the royal family were life time loans to be passed on to further ruling descendants. The prominence of the brooch may have fitted a life long loan especially if it was in the Queen Mother's keep from an early age. Somehow I believe that the sapphire brooch was returned to HM as was all of her mom's treasures on her passing. For instance the tiara most associated with Diana was also a loan and not a private/ personal holding.

Regards,

Dave.