On November 28th Intel released the Universal Windows Driver (UWD) for the Intel display (and audio). This driver now supports the new driver architecture for the 1809 build.
I installed the new driver and it fixed my issue! I re-enabled the Display Enhancement Service and my brightness remained the same.
I have the Intel UHD 620 graphics chipset.
Note: You have to install the UWD driver by running the .exe installer file. You cannot install it by selecting it with the have disk option for updating drivers. Download the driver from the Intel support site and follow the instructions from Intel.
So the issue was the the 1809 update was not compatible with legacy drivers, but the new UWD drivers were not yet available until just now.