dual external monitors: can't adjust brightness

Using two new 21.5 inch LG UltraFine 4K's (bought from Apple) as external monitors with a new 2018 15-inch MBP (2.9 GHz i9, 32 GB), and High Sierra.

With both monitors plugged into the MBP via the USB-C cables that came with the monitors, I cannot adjust the brightness on both monitors. The brightness sliders on the Mac (either via the Touch Bar, or in system prefs) will both adjust only one monitor, i.e., external monitor slider 1 adjusts the brightness on external monitor 1, and slider 2 also adjusts the brightness on external monitor 1, not monitor 2. (these monitors do not have physical buttons to adjust settings). The first external monitor that is plugged in is the one that can be controlled via the sliders. If the monitors are plugged in one at a time, the brightness is adjustable correctly. If the monitors or cables or the USB-C ports on the MBP are swapped, the behavior is unchanged (the first monitor plugged in is the one that can have its brightness adjusted, regardless of which cable, which monitor, or which ports on the MBP are used). Both monitors have their power supply cables attached; the behavior is the same whether the Mac is attached to its USB-C power adapter or not. Behavior unchanged after SMC reset and NVRAM reset.Thought it might be a cable issue so I was on the phone with LG customer service, and they are going to send me a new cable. Wondering if anyone has experienced this or has found a fix.