For me, I can't boot to system after each ROM update if I don't apply the patch... ^^
I don't remember if I applied the patch directly after formatting data/clean flashing in TWRP.
But... you can try this "Safe" conversion patch => TWRP_K20_to_Mi9T_v3_Safe_Conversion_for_Stable.zip
Info: Mount "System" partition only (no need to mount "Vendor" for this version).
What does the safe conversion patch not change in comparison to the "normal" patch?