Isn't that a recent practice that commenced when Prince Albert II was the Hereditary Prince of Monaco? The heirs to the Monegasque throne were known as Duke/Duchess of Valentinois for centuries until it became more common to title them Hereditary Prince/Princess. I think Hereditary Prince Albert was given the title Marquis des Baux instead of Duke of Valentinois because his grandmother, ex-Hereditary Princess Charlotte, was still alive and held the title Duchess of Valentinois at the time of his birth.
While the article does not state what its source is, it says the title "Prince Royal of Portugal, Brazil, and the Algarves" was not created until 1815, which is consistent with Brazil's elevation from a colony to a kingdom the same year, and the fact that the future Emperor Pedro I of Brazil was referred to as Prince Royal when he became heir to the Portuguese and Brazilian thrones, whereas his father was referred to as Prince of Brazil until he became Prince Regent (when Brazil was still a colony).
Here is Spain's announcement of the court mourning for José, Prince of Brazil (Pedro's uncle) in 1788.
https://www.boe.es/datos/pdfs/BOE//1788/089/A00714-00714.pdf
"Con motivo de la muerte del Príncipe del Brasil tomará esta Corte luto por 15 dias contados desde 24 del corriente."