Major Bug in 14291

Found a possible fix:

"You have the option to turn off UAC via registry by changing the DWORD "EnableLUA" from 1 to 0 in "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\policies\system". You will get a notification that a reboot is required. After the reboot, UAC is disabled."

I used this, then rebooted. I was notified once again that UAC was turned off even though I have notifications, all of them, turned off. But, this time so far UAC hasn't turned itself back on. Fingers crossed.

This does seem to be either new behaviour or a bug.  On my machines where I have UAC set to Never Notify, I also have seen this notification about UAC being off after being signed in for a few minutes.

If you just ignore the notification about UAC being off, then it does not re-engage.  It is the act of clicking on it that re-enables it.

Be aware that assuming you have been using the standard UI with the slider to set UAC to "Never Notify", this has not been turning UAC off.  Instead, UAC has still be on, but it just does not prompt you.  When you use the EnableLUA registry key, you are truly turning UAC off, which means you won't be able to use any UWP application such as the new Calculator, Edge, the Store or others as they require UAC.

Found a possible fix:

"You have the option to turn off UAC via registry by changing the DWORD "EnableLUA" from 1 to 0 in "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\policies\system". You will get a notification that a reboot is required. After the reboot, UAC is disabled."

I used this, then rebooted. I was notified once again that UAC was turned off even though I have notifications, all of them, turned off. But, this time so far UAC hasn't turned itself back on. Fingers crossed.

As JonathanKay has already advised...

Put the registry key back as you need it to prompt you when you need elevated admin rights and reboot.

To fix the issue of auto turning UAC back on to full after you turn it off try this procedure.

Right click start, control panel, Security and Maintenance.

Click - Change security and Maintenance settings

Under security messages - untick [   ] User Account Control, click [ok]

Click Change User Account Control settings

Move the slider to Never notify, click [ok]

Reboot and login again to check if UAC is still set to never notify.