LWJGL Issue
This report is invalid because it is a LWJGL issue affecting all numpad keys:
https://github.com/libgdx/libgdx/issues/2524
I am sorry if this is invalid because it is a bug with LWJGL or it is intended and I am also not sure if this is only happening with a German keyboard layout, but when you want to set the NUMPADENTER as the key for an action in the control settings it uses the normal ENTER
Confirmed.