My first guess was that it was because at that time I was on an Android 13-based custom ROM (Pixel Experience Plus 13 and CrDroid 9.0), but I was proven wrong after I flashed PE+12 shortly before trying MiFlash. Because it gave me the same error (Anti-rollback check)
Oh, I actually tried the flash-stock-ROM-via-TWRP method after I tried MiFlash method, so I doubt that's the issue. But all I can say is a few days ago I flashed 13.0.5 A12 firmware (latest at the time of writing), so could that be that issue?
I might wanna try what you said, thanks for the suggestion. But for now, I'm on EU and may stay for a few months
Always that I need as a base a stock ROM flashed, I "simply" use the fastboot commands that are inside the ROM, I open my ADB/fastboot CMD/terminal, and drag and drop the flash_all.bat file while in Windows or the flash_all.sh file while on Linux. This method allows to me, to can follow the process while the scripts are running, and search for issues (if there are some)
As per the TWRP flashing the stock ROM, it worked fine too. You just need to flash the ROM, then boot to recovery once, you'll find the stock one, then format data from it, then you are ready, either to go straight to system, or if you prefer have TWRP (or any custom recovery), go to fastboot and follow the method to boot to, and then flash it permanently to the ramdisk.