I have two Mi4c: one got the bootloader locked with that patch, the other didn't. I don't know why.
Luckily I had permission to unlock it, so it just took me a few seconds to solve it.
Could anyone provide functional links to the followings zips, especially libra_miui_fw_patch_MI4c_V9.5.1.0.NXKCNFA.
twrp-3.1.1-0-libra
libra_miui_fw_patch_MI4c_V9.5.1.0.NXKCNFA
PixelExperience_libra-9.0-20190324-0524-UNOFFICIAL
Most of the links are now dead and can't find the fw_patch_MI4c_V9.5.1.0.NXKCNFA anywhere.