Hello,
It should be possible to go from Stable (MIUI 11) to Weekly (MIUI 12) ROM because it's an update, but... you will get "only boot to TWRP" issue because latest weekly ROM for K20 Pro/Mi 9T Pro is still using an old "fingerprint build & security patch date" info from 2019-11-01 to pass SafetyNet test. And you can't "downgrade" the security patch date info.
So... you will need to install an updated "fingerprint build & security patch date", here is the latest from Global "Beta" MIUI 12 (If you have a K20 Pro, it may be recognized by Google as a "Mi 9T Pro" device, not an issue at all) =>
TWRP_RAPHAEL_12.0.0.8_Global_ROM_info.zip
But if you wish after that to go back (from Weekly to Stable), you must "Format Data" in TWRP.