Disabled then enabled kext signing; some keys no longer work properly
This is an issue with accessibility settings. The function is called "Enable Mouse keys". It is configured in Settings - Accessibility - Mouse &amp; Trackpad (left pane option), "Enable Mouse Keys" AND ALSO via the "Options..." button.

There is a checkbox setting "Press the Option key five times to toggle Mouse Keys". If this is active then simply pressing the option key 5 times will activate or deactivate this feature thus changing the functional mode of your keyboard.

https://www.youtube.com/watch?v=wxSo1xnxL_k