I have a similar issue on a Dell XPS 13 9360 that we have at home.
After the 1809 version was installed in October, the display brightness remained stuck at 50%. After the Nov Cumulative Update, the brightness could be adjusted but a new issue popped up as follows:
- If the laptop works on battery for a long time, then the display brightness remains again stuck to 50%: plugging to the AC power outlet does allow the brightness to increase. The only remedy I found out is to shutdown and re-start the laptop with a new start, then the brightness control works nominally again.