Same problem here with a Dell Inspiron 7537 and I never experienced this with earlier Ubuntu versions. The backlight automatically turns on in the following situations for me (regardless of the previous settings):
1. When I turn on or restart the notebook.
2. When I wake up the notebook.
3. When I don't use the keyboard and the touchpad for a few seconds.
Same problem here with a Dell Inspiron 7537 and I never experienced this with earlier Ubuntu versions. The backlight automatically turns on in the following situations for me (regardless of the previous settings):
1. When I turn on or restart the notebook.
2. When I wake up the notebook.
3. When I don't use the keyboard and the touchpad for a few seconds.