Well you can install the fastboot ROM. Try going from fastboot ROM to xiaomi.eu using the official guide from p.1 to the letter with official TWRP only if it doesn't ask you for password to remove encryption use the recovery from geoanto555. I have used it when I had problems with decryption and it has been working great. Also do MD5 checks on the zips you are flashing. I have to mention I have never had official MIUI 12 image flashed on my RN8 it could be the case that official TWRP doesn't work with it.same problem
It came from the rom 20.11.25, everything working fine, I installed 20.12.9 and the TWRP lost the ability to decrypt
So I stayed there, I found that they published the solution for twrp-3.5.A9-ginkgo-geoanto555.img so I flashed it and could decrypt it, so flash this rom
now my phone only does a bootloop to TWRP but everything is encrypted and does not ask for a password.
flash the TWRP again, flash vbmeta.img but it remains at the same point, encrypted and in bootloop to TWRP.
download the rom ginkgo_global_images_V12.0.3.0.QCOMIXM_20201226.0000.00_10.0_global_044c052090 and install via MIPCSUITE fastboot and only then have I managed to start.
but if I flash again this rom or even 20.11.25 remains in bootloop to TWRP.
now i can't install any rom without getting to bootloop,