XPS 15 9550 (4K Display) Brightness level

Every time I restart my laptop (this includes sleep/wake up) the brightness level of the display is restored to an "unknown" value (always the same, though). I have tried changing all brightness level settings in all power plans, I disabled the adaptive brightness in advanced power settings - and some other values there - but to no avail. When the laptop restarts/wakes up it stays a few seconds at the desired level, then it drops back to the same value (always 49%). After this, If I manually change the brightness level, it stays only until the next restart/wake up.