The simple answer is: she has no possibility to wear it. At State Visits Princess Claire will always wear the Order she has been given by the guest. On his turn the guest will wear the Order he/she has been given from the Belgian King.
The Belgians simply do not know events like in other monarchies where the own Order can be worn by ladies in long. For an example the northern neighbours have the annual Prinsjesdag (picture) or had the King's Investiture in 2013 which saw the Dutch royals wearing their Dutch Orders indeed (picture).
On a state visit, the guest is likely to award a foreign order to the King and maybe to his heir, but not to the wife of the King's youngest son. In that case, since she doesn't have a foreign order of her own, Princess Claire would wear the highest Belgian order she has. That should also have been the case at Victoria's wedding as Claire apparently has never received a Swedish order.
In other words, the only plausible explanation for Claire not wearing the Order of Leopold at Victoria's wedding or the Finnish State Visit is that she somehow didn't want to wear it.
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