The problem:
- Unplug laptop power supply
- On screen energy saving in settings
- Uncheck screen brightness (on all profiles or on that will be loaded)
- Wait minutes display to do off (not dimmed)
- Plug power supply without display on
- When it's on, screen brightness is 100%
Probably it's depends of backend (intel in my case) on dpms it returns *garbage* values even on max brightness (it's still not tested)
The solution:
Trying to cache max brightness level and denied to change screen brightness on garbage values