This is really crazy. I have ASUS UX ultabook and since 1809 I have 2 problems. One is SIMILAR to this. The difference is that the slider is NOT at 50%, the slider is at 100% but the brightness is at approx 50% !!!
I tried updating the driver for Intel, NVidia, I even installed insider's preview of Windows but the issue remains!
The thing is that when I turn the notebook on, the display is at 100% on lock screen. As soon as I logon, it is reduced to like 50% and it can be adjusted, but the maximum brightness is not 100%!
The other problem is that sleep mode doesn't work, but that is for another topic...
I'm experiencing this exact issue (Windows says 100% brightness, brightness is actually 50% when on battery) with my HP ZBook 14 (Gen 1). A reboot sometimes fixes the issue, but that's barely an acceptable workaround. I have installed the latest BIOS and display drivers from HP.