I've experienced the same unable to decrypt error in my 11 Ultra. I've been in weekly fastboot installation of EU room. I've stopped updating after encountering this issue on a recovery EU rom I think started on June 22. Now that I've time, I tried exploring it again and this is how I resolved my twrp not decrypting data issue.
Step 1. I've downloaded old fastboot EU rom (I've used May 13, 2021 version), then execute
C:\Users\digeratz\Desktop\eu_rom\21_5_13>
windows_fastboot_first_install_with_data_format.bat
Step 2. After the installation is finished, wait for it to boot. Be patient, it takes time. (note: if it goes to MIUI recovery mode with the wife data option, just select wipe data and reboot to the system, you again need to be patient to wait until it boots to the system)
Step 3. Once you're in MI12.5 system, set up the phone with your pattern lock screen passcode.
Step 4. connect to PC then copy the latest EU zip recovery rom into the phone
Step 5. Turn off then go to fastboot mode. You can now see the fastboot version with the MI bunny.
Step 6. execute fastboot boot twrp.img (I've used the latest - 3.6.2)
Step 7. You should be prompted with the pattern passcode. input your pattern and it should now be able to decrypt the phone.
Step 8. Voila! you should be able to install the latest eu rom via TWRP.
I think the decrypting issue happens to those early EU rom users. When the time we need to use the 3.5.1 modified chinese TWRP (not official for 11 Ultra) then after that the fastboot versions were released. As we do the weekly update, you'll noticed that the fastboot mode was changed too, from the one with the MI Bunny to just the FASTBOOT (without the bunny). I believe somewhere in this process, something was messed up affecting the decrypting of data.
Now, by bringing back the MI bunny in the fastboot then flashing the new EU recovery rom via TWRP, helps resolved my issue. Hope it will help you too.