Brightness function keys stopped working

[o0okrishnao0o]

Distributor ID: Ubuntu
Description: Ubuntu 18.04.3 LTS
Release: 18.04
Codename: bionic

The function keys that raise or lower the screen (backlight) brightness have stopped working. Not completely, using the keys brings up the brightness level indicator pop up, but the level (which is stuck on max) will not change. Adjusting the brightness via other methods (e.g. battery system tray icon) does work.

There is a similar post already existing and this makes me wonder if the brightness down function has been remapped somehow to brightness up. This would explain the current behaviour.

How do I view/edit keymaps?

Nvidia GPU
Intel CPU

[o0okrishnao0o]

It's not the key mapping. Something else is wrong. When the brightness is set low the function keys do not raise it either.