Unable to Remap the TAB Key From "Scoreboard"

I am completely unable to unbind the TAB key from "Toggle Inventory / Scoreboard (Hold)" even if I completely clear the keys, as it appears to be hardwired.

That means if I try to bind my "crouch" key or "big map" key to TAB and my "Toggle Inventory / Scoreboard (Hold)" key to something completely different like CRTL, it STILL registers it as a "duplicate" and in game when I press TAB, the scoreboard still comes up. Even if I hold ESC and completely clear all the keys and then rebind them, it still will not unbind TAB from "scoreboard." 

This is making the game unplayble for me... Please address it immediately. Something this basic should not be an issue in an $80 AAA title game.