rooting - What is CSC Fastboot Mode, and how to get rid of it?

I am trying to access my Zenfone Go's fastboot in order to unlock its bootloader, but it seems to have been replaced with something called "CSC Fastboot mode".

In this mode, I just see the ASUS logo, a "CSC Fastboot Mode!" text, and I am unable to enter any input. It looks like this:

Since I cannot enter any input, most communication with fastboot.exe is impossible. I somehow managed to have fastboot.exe correctly detect the device, but any command that requires confirmation on the phone is impossible (ex: fastboot flashing unlock), fastboot.exe just waits forever. I tried inputting the commands on the phone anyway, in case there was an invisible menu or something, but didn't work.

What I tried:

• 
  

  Turning off my phone, then "Power Button + Volume Up Button" until it restarts. It goes directly to CSC Fastboot Mode.

  
  


• 
  

  Turning off my phone, then "Power Button + Volume Down Button" until it restarts. It goes into Recovery Mode. Then I select "reboot to bootloader". It goes to CSC Fastboot Mode.

  
  



• 
  

  Accessing my phone through adb, and using the "reboot-bootloader" command. It goes to CSC Fastboot Mode.

  
  

What is CSC Fastboot mode? What is it caused by? And how to reach a usable fastboot menu?

Model Number: ASUS_X013DB

Android Version: 5.1.1

Build Number: LMY47V.JP_Phone-13.0209.1705.116-20170621044040677_2015122901270020000400001