I can tell that indeed the in-game button to enable and disable headlook is not bound to anything for me any more since i use an eye-tracker. Unfortunately i do not use an x-box controller but fly with HOTAS. Binding a key from either the keyboard or from the HOTAS to enable and disable ISE works well for me, so i can’t tell what your issue could be.
Although, just to be sure, you have set the Enable/Disable key to be set to toggle? I know you almost certainly have done so, but i still think i should ask.