Unlock bootloader at Meizu pro 5

Hi I have meizu pro 5 and I have firmware with root, I have try to unlock bootloader i do so:
1 I go into fastboot mode and i read (locked,root)
2 then i do fastboot oem unlock
3 then i do fastboot reboot
4 After that smartphone is reboot, i do adb reboot fastboot and i read (locked,root)
why i can’t unlock bootloader ?

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