In Developer Options, OEM Unlocking is grayed out (disabled control), with radio button Off - but I think it was also previously like that, when I was running TWRP+b378/b383.Ok, in this case, you might have one more thing to try. Lock back the bootloader (relock) and then unlock it again. Even if this time it fails to flash then it might be something with the new version of your OS.
How does it look for you "OEM Unlocking" looks for you (grayed out, button in Off position, or?)
Should I still try un-locking/relocking Bootloader with this position of OEM Unlocking control?
Another option I was thinking about - if somebody can send me a pre-rooted (based on SuperSU-v2.79-201612051815-EMUI5.0 ?) boot image for l09 c432 b386, I will try to flash by:
fastboot flash boot b386_boot_stock.img
just to start with b386 rooted (while still trying to flash TWRP)