For those who originally bought the game on PS4 back in 2020, there was additional DLC to it despite being the Royal version. Last year, ports were released for all other platforms which included all the Royal DLC, but those with the original PS4 version were left behind, with the DLC still requiring a purchase.
This now seems to be rectified - the PS4 versions' DLC is all free at the moment. Not sure if this is a price error, but while it's available, if you only have the original PS4 version, might as well snag it while you can.
I should note that it does seem all the non-DLC versions of the game have been removed for purchase too.
probably doing this since all new releases (switch, pc etc) include all the dlc