Finally managed to unlock the bootloader on a recently bought Poco F6. Now you have to install an app asking the CCP to get a license to unlock your device on top of using the MI unlocker and wait 3 days!
No more Xiaomi phones for me. 
Discussion
Do you update to Hyper OS 2 before to unlock?
Which is the app?
No need to update the phone before unlocking.
Read this tutorial on XDA forums for a detailed method on his to unlock Xiaomi Phones:
https://xdaforums.com/t/how-to-unlock-bootloader-on-xiaomi-hyperos-all-devices-except-cn.4654009/