I looked into this just now since upon checking I have the same firmware version 12.0.6.0 and I found that there is a much updated stable release 12.5.6.0 in sourceforge. I downloaded xiaomi.eu_multi_HMK20ProMI9TPro_V12.5.6.0.RFKCNXM_v12-11.zip and installed it just fine and immediately went back to you. I registered just to post this comment and I hope it helps.
Note that even though we are using K20 Pro Premium, there is only K20 Pro firmware available and it works for us.