If people in positions of authority aren't going to speak out against dehumanization (slavery, the Indian caste system, the Holocaust, Jim Crow, apartheid, brown kids in cages at the border) then they shouldn't BE in a position of authority. I understand what everyone has written about how the Monarch can't express his/her opinion in public, but IMO it was because of that prohibition that the Queen's leaked opinion held true power.
Additionally, she was acting not in her position as Queen, but as the leader of the Commonwealth. I think there's an important distinction there. The problem was that the ONE who was fighting against the 48 other members happened to be the PM of the UK.
I realize this is only my opinion and I can't change the world, but apartheid was a crime against humanity and the Queen's push for sanctions even if outside the "rules" was an important piece of the puzzle that helped to change an abhorrent policy.