Well, I cannot for the life of me install any ROM but official Global on my Mi 10T Pro apollo_eea_global_images_V12.0.18.0.QJDEUXM_20210109.0000.00_10.0_eea. I have installed many ROMs on phones in the past, but this is beyond me.
My best bet was to use xiaomi.eu_multi_HMK30SUltra_MI10T_V12.0.3.0.QJDCNXM_v12-10-fastboot.zip that was published by xiaomi.eu a while back, using the windows_fastboot_first_install_with_data_format.bat
I tried that without installing TWRP or magisk or anything else and while the install went through without errors and it got past recovery when booting, it resulted in a bootloop.
Installing twrp-3.5.1.A10-apollo-geoanto555.img and flashing Disable_Dm-Verity_ForceEncrypt_11.02.2020.zip didn't help.
Installing PBRP-apollo-3.1.0-20210404-1937-OFFICIAL.zip (Pitch Black TWRP based on 3.5.1) will always result in booting to recovery so it is useless at the moment.
I can always flash the official ROM back via the MI Flash tool and it boots up but any attempt to install another ROM fails.
If anybody has some pointers beyond what I already tried I would be very grateful!