Just to check if you can still boot into twrp, because there is a warning for "mi10 pro users not to boot into twrp after flashing 20.15.10. Are you also using the "mi 10 pro twrp" on your mi 10 ultra?
Thank you so much for adding the mi 10 ultra on the devices supported by miui eu! Unfortunately I also have the windows issue with the .bat script. Waiting for twrp or a fix to the windows script as I have no access to linux or macOS.