Surface pro 3 Brightness control missing

Well... this is awkward.   I did a hard rest (thinking the image stored on my Surface Pro 3 was Windows 8.1),   and I actually ended up reinstalling Windows 10 again.   Upon boot, the Brightness worked.      

I've made it his far before, so then I began to apply updates.    And, the brightness is still working (12 hours later).   

I've seen this before (where it worked for 2-3 days after an reset), so I don't want to get my hopes up. 

Still,  I'm cautiously pessimistic that by going back to an OLD version of windows and jumping to the MOST RECENT Roll-ups, I've somehow sidestepped whatever patch or configuration issue caused the problem.    I'll report back in a week.  

I've done complete resets and reinstalls several time before, so this is the very definition of insanity (Trying the same thing and expecting different results).    I can't imagine what's changed, except that the path to the current version of Windows bypassed some of those in interim builds.   

Perhaps there was a minor defect layered into an obsolete minor update that can't be remedied by subsequent updates... but the defect can be avoided by leapfrogged from an old build directly to the most recent 1909 Roll-up Package.   

I have no idea what to think anymore. I've tried 14 solutions over 18 months including two other resets.    I just hope the problem stays "gone".    I've also got back my power management settings, and my power/sleep  button is still working.   So, so is the magnetic keyboard trigger to go to sleep when the keyboard is closed.    None of that worked after a few days in prior resets.    

Anyway, I'm currently running:

Edition:  Windows 10 Pro

Version: 1909

Instaled on:  8/4/2020

OS Build: 18363.959

On a Surface Pro 3 with an Intel i5-4300U CPU at 1.9ghz 2.50Ghz and 8Gb Ram.

What you describe has been my experience also.  It is so difficult to troubleshoot when there have been multiple updates and applied patches.  For now, I don't have the issue but it may reappear after the next Windows update.  I will more than likely replace this Surface Pro 3 at some point soon and have moved most of my apps to my desktop device (iMac running Bootcamp).  My Surface has been relegated to mostly reading books and watching movies!  Thanks for the input.

I just had this problem and it was fixed by uninstalling Surface_Brightness_Control

That's exactly what is happening to my SP3. I can see the slider but it will not allow any adjustments. The slider will not even move.

All the problems can be solved when you install the MonitorDDC app. The prog can detect my SP3 screen (120QL01) then provide ability to change the LCD brightness (and show brightness level in real time if you or other softwares change it). It also provides features of rotating the screen and assigning keyboard shortcuts to a variaty of screen and volume changing.

The app's link: https://clickmonitorddc.bplaced.net/