For your info, don't flash any AOSP rom over OneOS.yep my bad, I am trying right now after having flashed stock 12.5.2. I had trouble with mi flash tool and my ryzen PC, FYI I had to use an intel PC to succeed flashing fastboot rom. I am suprised I got stuck and could do anything via twrp, I never had to stock flash my mi9 after messing with custom roms (android 9 and 10), I loved using revolution OS on it !
Edit : Installation worked after installing stock rom first, fastboot 12.5.2 via mi flash tool