After seeing task T7368, I knew the reason that make my Protection banner (riped-off from Wikipedia) did not work properly for my wiki.
In my module's config, the padlock and the banner only work when the protection level were extendedconfirmed, not editextendedconfirmedprotection.
See this page to know what happen to my page when I protected the page as editextendedconfirmedprotection without the small = 1 option.
As you would see, the banner (in Vietnamese) still displaying the Full Protection with the golden padlock - which were the default one, not the Extended Protection with the blue padlock. (Hover your mouse to the padlock on the banner)
(quoted comment for reason why I created this task: