Surface pro 3 Brightness control missing

This is a little unclear. When you open the Action Center, the brightness
control is at the very bottom in 1903. Are you saying the slider does not move
or that you can move it but the brightness does not change?

I have the same issue.

The brightness slider in action center is disabled. you can hover it with the mouse but you can't move the slider.

When using hotkey combos (FN+Backspace, FN+Del) same result, no changes.

This has started since the 1903 W10 upgrade.

I tried reinstalling the display driver, removing it, etc. didn't work.

I also tried using Windows Mobility Center, which provides a handy brightness slider, the slider works but it has no effect.

With this problem, you can't keep the surface on battery because it drains really fast at 100% brightness.

I tried reinstalling the display driver, removing it, etc. didn't work.

Did you download SurfacePro3_Win10_18362_1902002_0.msi from
https://www.microsoft.com/en-us/download/details.aspx?id=38826 and then
uninstall and DELETE the Intel video driver, run the MSI and restart?

Yes I did. Nothing changed.

I'm afraid the only "solution" I can ask you to try if you are beyond the 10 day
roll back period for feature updates is to wipe and reimage from a downloadable
recovery image. You will need to back up any files stored locally and be
prepared to reinstall apps and programs. Go to
https://support.microsoft.com/en-us/surfacerecoveryimage   (with a different Windows computer if yours is not working properly)  and download a recovery image for your Surface. You will need a 16GB USB stick
formatted FAT32 (use a Windows computer). Use FULL format and not quick format
and give the volume a name like surfboot.. Best to use a new previously unused
USB stick for this. There are full instructions and a video at
https://support.microsoft.com/en-us/help/4023512/surface-creating-and-using-a-usb-recovery-drive#download
but below are quick instructions. Unzip/extract the recovery zip to the formatted USB stick and then-

To reset your Surface from a USB recovery drive:
Step 1:    Make sure your Surface is turned off and plugged in (hold down the power
button for 30 seconds).
Step 2:     Insert the USB recovery drive into the USB port on your Surface.
Step 3:    Press and hold the volume-down button while you press and release the
power button on your Surface.
Step 4:     When the Surface logo appears, release the volume button.
Step 5:    When prompted, select the language and keyboard layout you want.
Step 6:     Select Troubleshoot > Reset your PC.
If prompted for a recovery key, select Skip this drive at the bottom of the
screen.
Step 7:     Select the target operating system you wish to reset. This
refers to the current operating system installed on your Surface.
Step 8:    Select Yes, repartition the drives, and on the next page, select Next.
Step 9:     Choose Just remove my files.
Step 10:    Select Reset.
Surface restarts and the Surface logo displays while the reset process continues
(this can take a while).