Aha, it turns out to be my hardware failure: I connected a triode and an LED on the wl_reg_on, trying to use which to indicate the wl_reg_on is on. But at the same time, the triode and LED makes the wl_reg_on is always low. I solder them off, and ap6212 works fine now.
By the way, my sdio wifi is still recognized as mmc2. I bought a orangepi zero plus 2 a few days ago, and it recognized sdio wifi as mmc2 too.
Much sorry for my hardware mistake and much thanks for your patience and help!