My guess is the latter.

QM probably thought that Queen Elizabeth, being Scottish herself, would have suitable Scottish jewelery so she left it to Lilibet.

Remember how QEII did not realize that the diamond bandeau given to her as a wedding gift was actually the base to the Girls of Great Britain and Ireland Tiara - she probably inherited the Thistle, along with so many other items that she just forgot, or was not aware of, it until she sent Angela Kelly onto to the vaults where it was "re-discovered"

......just a theory!!