Maybe it depend to the hardware, because they can slow down the speed of some devices with lighter hardware? On my Mi 11 Ultra all super wallpapers are displayed. I´ve hoped for new ones, but only the old are within the thems app. Otherwise try to choose them over a long tap on homescreen, not the themes app. This helped for me, because after inbstallation was the themes app empty. After trying this method, the themes app displays all items as before.