I think that Stephan is correct: the last tiara occasion for which we have photos is from the Diplomatic Reception in 2019.

Although we know from H Roberts that The Queen was given the Halo tiara for her 18th birthday, I haven't ever seen references to her wearing one before she married. But, I have no doubt that she would have worn a tiara on some occasions -it is just that the information wasn't given to the press.