In my case (Dell XPS 13 9360 with Intel HD Graphics 620 - Windows 10 1809 Home 64bit) the proposed step to disable and then re-enable the device restores indeed the brightness control, but only for a limited time.
In fact, continuing to work with the laptop on the mains current and then for a few hours on the battery, the brightness control goes down to the 50% level as set for working with the battery, but then it gets stuck again. (actually, it is not a big issue
for this specific laptop for me, its screen is extraordinary bright, so also keeping 50% is it good enough) .
Another simple solution (at least in my Dell XPS case) is to restart (New Start) the laptop - then the brightness control is restored, but always for a limited time if one swaps between mains current and battery.
For this Dell XPS laptop, there was indeed an improvement after the November/December (? - I am not sure) cumulative update of Windows 10 , the brightness control could be again regulated , while with the 1809 version as originally delivered early October
it could not, remaining fixed at 50%, but, as said, the brightness control now is only possible for a limited duration of work .
I know that Intel has delivered in Dec updated graphic drivers,
Windows® 10 DCH Drivers'. For the time being in my specific case I prefer staying with the driver as distributed by the PC manufacturer (Dell in this case). Should the problem persists in the coming months, I may install the new Intel
drivers, downloading them directly from the Intel website.
Just for information: I have also at home two relatively old laptops (ASUS N76VZ) running Intel HD Graphics 4000 plus Nvidia Geforce GT 650M - they were not affected by the brightness control issue of the Windows 1809 early October delivery.