Problem Details:

As long as Toggle Scoreboard is bound to Tab, both Toggle Scoreboard and Toggle Interface work as intended. If however Toggle Scoreboard is bound to a different key (tested with Decimal on Numpad, no key binding conflicts), you are neither able to toggle the interface with Shift + Decimal nor with the original Shift + Tab, it just doesn't work.

Furthermore, Decimal will toggle the scoreboard correctly, but holding Shift and pressing Decimal will do nothing at all (it locks it for some reason).

Going back to Tab for Toggle Scoreboard is the only solution I found yet.

Tested on both Stable and CuttingEdge.

I can confirm this. This is an actual issue. I think it should be considered.

Seems like having it bound to any numpad key will make you unable to toggle the interface by holding SHIFT and pressing the key. It works fine if the Toggle Scoreboard key isn't on the numpad though. Confirming...

Edit: Moving to low priority