I have no problem with Desire 20 pro and have no guess why it doesn't work with U20
C:\adb>adb reboot bootloader
C:\adb>fastboot devices
CN06D1V02*** fastboot
C:\adb>fastboot flashing unlock
...
OKAY [ 0.040s]
finished. total time: 0.042s
guess, full working S.ROM for m4 would be possible with overcoming the kernel issue
I've got the ROM by flashing over the [ROM][4.4.3][Sense 6] LIBERTY ROM with 12.13 release
or with a previous release in manner I mentioned above (build.prop edition)
tried it several times with the same result
UPDATE:
got the rom booted
flashed over [ROM][4.4.3][Sense 6] LIBERTY ROM - The Sixth Sense, with edited build.prop
ro.build.version.release=4.4.3 to 4.4.2