Surface pro 3 Brightness control missing

Update on this:

Plugging and unplugging an external screen will reenable the settings for brightness so that you can at least interact with them, however as soon as you try to actually move the slider it doesn't in fact change the brightness and the settings window crashes.

I'm having the same issue (brightness slider in right hand side disabled, the setting "automatically adjust brightness" just vanishes from the settings>display options, as well as my power button no longer putting the surface to sleep.

However I've realised that when I plug another monitor in so that my display is extended, the settings come back and I'm able to adjust the brightness of the surface pro screen. As soon as I unplug the additional display, all the surface brightness options are disabled / removed. 

The power button still however doesn't put the surface to sleep.

Is anyone else experiencing this behaviour?

Same problem here!

Also my power button does not turn off the screen!

And also the screen does not go off when the surface is folded on the keyboard like it used to.

This is absurd one would expect outstanding stability from a Microsoft own device after so many years from its release

My SP3 got the exactly same issues after updating to 1909. I've tried most of the solutions mentioned in this thread, and none of them worked. Resetting PC with keeping personal files was unsuccessful for unknown reason. 

Besides, I tried the "Surface Brightness Control" tool (https://docs.microsoft.com/en-us/surface/microsoft-surface-brightness-control), which is designed to "lower the overall carbon footprint" by auto dimming after some predefined inactivity period. Sometimes it can dim my screen to 20%, but most of the time it's not functioning. (I had to create the registry keys by myself, but it's still not working as expected after tuning the values.)

Finally, as suggested by this post: https://www.surfacetablethelp.com/2019/12/brightness-slider-control-missing-in-windows-10-on-surface-pro-3.html, I tried the third-party tool PangoBright. Win 10 is not mentioned on its induction page, but surprisingly, it works. It can adjust the brightness in 10% interval from 20% to 100%, which is good enough for me. (Seems it conflicts with "Surface Brightness Control", so that tool should be uninstalled.)

However, PangoBright is not perfect. The brightness still goes to 100% when showing the start screen and the side bars. And other problems (empty Power and Sleep setting, power button doesn't turn off screen) still exist. For now I have to live with this until Microsoft fix this stupid bug.

Update:

After using it for a while, I realize PangoBright doesn't change the screen brightness actually, but just change the color for all applications. So it cannot save any power consumption by LCD. 

In my last attempt, I managed to reinstall Win10 using the image downloaded here, which still allows me to keep my files. I don't know what happened, but after reinstallation everything looks normal: the version is 1909 and auto updating seems don't break anything.

I have the same problem. I also tried disconnecting the type cover, reboot and change. But I still can't change then. I have the Surface Pro 3 i7 8gb

I also tried the following methods:

https://www.tenforums.com/tutorials/70157-enable-disable-adaptive-brightness-windows-10-a.html

Option 1: those options aren't available

Option 2: Doesn't work

Option 3: I did this one, but didn't change anything

Option 4: I can't do this because I because there is no Intel Graphics Settings, I tried to install the Intel drivers manually but I can't install them (https://downloadcenter.intel.com/download/28662/Intel-Graphics-Driver-for-Windows-15-40-?product=75114)

https://surfacetip.com/how-to-unlock-power-plans-on-surface-devices/

I changed CsEnabled to 0 in regedit, but that also didn't help

I now do have more power options and I can change them (but there is still no adaptive brightness option). But I can't choose any of the power plans (it gave an error)

https://windowsreport.com/adaptive-brightness-windows-10-wont-turn-off/I did the 5. Disable Adaptive brightness in the Registry but that also didn't help

This is really annoying. I don't want to use adaptive brightness, manual is fine with me. I just want to be able to adjust the brightness. Even on battery saver the brightness looks like it is at full.