Today MATE decided to take NumLock serious. It's settings manager was unsure what state of NumLock was desired and thus changing it over and over. In that case, typing on a notebook can be rather tedious as you happen to press the keys in order to get a specific character and not any out of two possibilities. There is a two step solution. With
you are able to control NumLock again. The next thing to do is to fire up dconf-editor (part of the package dconf-tools) and set org.mate.desktop.peripherals.keyboard.remember-numlock-state to off. Now any forgetfulness of the settings manager will not affect your numbers any more.