Adjust brightness of LG Ultrafine 23" display

How can I control the brightness of the LG Ultrafine display when connected to my macbook pro? when I first purchased the display the brightness control in system preferences would only show up on the LG display - I could control brightness with bluetooth keyboard for the LG display, and I could control brightness for the MBP with the keyboard on the MBP or System Preferences.

Now, the brightness control is only available for the MBP screen, and the bluetooth and MBP keyboard controls only change brightness on the MBP screen.

Anyone know why this might have changed, and more importantly, how to control brightness on the LG Ultrafine 23"?