It looks as though LWJGL doesn't "understand" the 5 key in the numeric keypad when numlock isn't on. When Numlock is on, the key generated is KEY_NUMPAD5, but with numlock off, LWJGL sees KEY_NONE, which doesn't strike me as right.
AWT generates the KeyEvent with "VK_BEGIN," if that helps.
The other numpad keys can be caught, although it looks like they're indistinguishable from their non-numpad equivalents.