LG UltraFine 5K Display dim each time I startup

This is happening to me to with the LG 5k Ultra Fine. Talked with a senior Apple Technical Support rep a few months ago who, I believe, is still investigating the issue.

Has anyone else found an issue? This is VERY annoying to say the least.

In my case it is the opposite. When I connect a late 2016 Mac Pro 15" (OS 10.12.6) to an LG 27" Ultrafine it usually turns on with max brightness. I have done the NVRAM and SMC reset to no avail. Something is not set correctly in system brightness settings.

I just want to add to this thread- I have the same problem. I have a maxed-out late 2016 (touch strip) MBP with the 256GB storage, 16GB RAM, Premium Graphics Card, i7 chip. Since the latest software update my 27" LG 5K monitor doesn't remember its profile and turns on at the lowest brightness post reboot. (My MBP was bricked upon the installation of the latest OS X). The Geniuses erased my MBP and successfully installed the new OS X but it now had this problem with the LG monitor- Took it in again and now they sent it off for a new logic board. I'll let you know what happens..

For what it's worth, I've posted a prior reply stating that I have the same problem. I did both the NVRAM and SMC resets per instruction from the tech support at Apple Care. I took my MBP and the "27MD5KA" along with my Magic Trackpad and Magic Keyboard to the Apple Store and they witnessed the problem. I suggested to swap out the cable/ monitor with a floor model but they claimed it had to be the computer hardware. Apple subsequently replaced the logic board and the i7 chip. Unfortunately it still has exactly the same problem(s). Oddly it will sometimes reboot with what seems to be the previously-set brightness, etc, but it's not consistent. Also, during reboot, there are many graphics antics and temporal delays for various things such as "shake to enlarge cursor" and recognition of the bluetooth keyboard/ trackpad. (It can take about three minutes for the "shake to enlarge cursor" function to operate which is very inconvenient). I hope this helps. Let me know if you learn of any clues please as I can't rely on the setup I paid for to perform.

As a workaround, maybe the brightness can be set programmatically at startup with a script somehow? There is a command line utility that might be able to do that, but I haven't tried it:

GitHub - kfix/ddcctl: DDC monitor controls (brightness) for Mac OSX command line

I now suspect this brightness change has to do with the Night Shift setting. If I have Night Shift on the problem occurs, if Night Shift is turned off there is no brightness change issue from one day to the next.

IOW, if Night Shift is on and I put the machine to sleep and later return to it, likely after the Night Shift cycle ends, I find the brightness at max.

I am also having this issue and can't get the brightness to increase on the monitor at all

Makes 3 of us now. My LG 5K was perfect yesterday. Fired up my mac this morning and it almost black. I can see the apple menu but the monitor is basically almost black.

OK found out that something is causing the brightness setting in system settings to be dropped to zero for the LG5K. Seems odd but has now happened twice.

Same here. But what is causing that to happen?

I plan to open a support ticket with Apple this morning.

Still unresolved as of Nov. 25, 2017.

My 2nd monitor is also not bright enough sometimes and there is NO fix that I can find that works. Lots of rabbit holes, but no fix.