My redmi had a vendor ROM. The unlock bootloader was unofficial.
Use your mi account to ask for unlock authorization from offical site. You will get a SMS with a code to confirm your request. After that, wait a few days to get the SMS with permition.
You must install latest official development ROM (it can be global, fastboot ROM, not zip). Install it with latest xiaomiflashtoll. Put your phone on edl mode (be careful with drivers, you must have qualcomm driver installed).
Google it to find out the right detayled steps
After the instalation, you must use the offical unlock tool after, because your phone will be with bootloader locked again.
After the unlock, install latest twrp and inside twrp do a factory reset. Install the latest ROM from miui.eu site and supersu (zip files).
Wipe cache. Reboot it.
Done.
It worked for me. It looks like that bootloader unlocked with unofficial method was the problem.