I am not familiar with that particular model, but I think all Chromebooks work the same.
When you boot up a Chromebook from a powered-down state, the keyboard backlighting is in an automatic mode. It will turn on or off based on room lighting, just like you describe.
BUT
If you manually adjust the lighting level with the keyboard shortcut, you are then in a manual mode. It will retain that brightness level until you shut down the Chromebook.
SO
Have you tried to simply shut down and reboot?