Did you find a fix?
I never had so much trouble getting TWRP to work
I did the same upgrade to new TWRP and PIE with a Redmi 4 Prime (Markw) and it took me 5 min to succeed. But when I tried with updating TWRP on my Mi4C (Libre) it would not boot into new TWRP. I have spend hours with no luck. I tried several offical and non-official versions of TWRP for Libre. They all show success when flashing TWRP both from within TWRP and with adb/fastboot. But then won't boot to recovery, but hangs at Mi logo. I originally came from twrp cofface from Teamsuperluminal, which I believe was around version 2.8.
I tried flashing all newest officiel from AndropaX (twrp-3.1.1-1-libra.img, twrp-3.1.1-0-libra.img)
then from other developers and lastly:
twrp-3.2.1-0.img (Vovan77)
TWRP+3.2.3-0_libra.img (Vovan77)
TWRP+3.2.2-0_libra.img (Vovan77)
As mentioned all succeeded with no errors, but then would not boot. However from fastboot/adb I could flash, but not run fastboot boot <img file name>. It would give me the error: FAILED (remote: dtb not found).
I did manage to get TWRP 3.1.0.0 (twrp-20170815-0837.img from Cozzmy13) to boot with no issues, but when inside TWRP it gave me error 7 when flashing. I then tried to remove the first lines starting with assert from the custom rom and TWRP was then able to flash the ROM with no errors. However now the ROM did not boot

Tried with both PixelExperience_libra-8.1.0-20180825-0823-STABLE & lineage-16.0-20190102-UNOFFICIAL-libra)
I checked my bootloader and it is not locked and I was still able to restore my nandroid with TWRP 3.1.0.0.
Really want to upgrade to PIE and don't want to give up. Hope someone found the fix.
At the moment my last guess is that I new to update my bootloader somehow - indicating from the errors I received. I believe the current was marshmellow. To do this I need to flash latest miui? Meanwhile I'll try to flash xiaomi.eu_multi_MI4c_V10.1.1.0.NXKCNFI_v10-7.0.zip and see if it updates the bootloader.