Why Can We Not Rebind Keys To The Number Pad?

I dislike the default key layout and have used a custom one that also makes use of the keys on the number pad. Rebinding them has worked fine from TSW2 -> TSW4 but was removed/broken on TSW5 and stopped me playing that iteration and now also TSW6.

When you try to rebind keys the game registers the input but when to click apply it sets the bind to none.