I remember that the Cartier halo tiara, Kate's wedding tiara, was loaned to a Cartier exhibition abroad at the time of her first State Visit attendance. The halo tiara was simply not available for her to wear at that time, so she wore the lotus flower tiara instead. And we don't know if she had access to all three tiaras at the same time, or if she only has Queen Mary's lover's knot tiara now, and the other two are back in the Buckingham Palace vault.
Time will show if the tiara distribution policy of King Charles will differ from that of his mother. The BRF certainly has enough jewels to deck out all the working royal ladies. But will they want to flounce it more?