One more question and something to try:
When you power on the laptop in the morning, if you look at the screen brightness slider in Windows, is it noticeably in a different spot than where you last left it?
As I think about this, here is one more thing to check: Do you have Fast Startup enabled in Windows? If so, please try disabling it and see if that by any chance resolves the issue.
Turn On or Off Fast Startup in Windows 10
Yes, there is... but that doesn't help unfortunately.
I watched what happened using Process Monitor (ProcMon) whilst I slid the Change brightness slider (Settings > System > Display > Brightness and colour) to 100% down to 0% and back again.
ProcMon identified the registry key easily and showed it to be buried deeply. On my device it is here:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Power\User\PowerSchemes\381b4222-f694-41f0-9685-ff5bb260df2e\7516b95f-f776-4464-8c53-06167f40cc99\aded5e82-b909-4619-9949-f5d71dac0bcb\aded5e82-b909-4619-9949-f5d71dac0bcb\ACSettingIndex
The key is configured so only the System account has Full control. Other accounts only have Read permission, so cannot change the data value of the ACSettingIndex entry. The CUE app must therefore be elevating itself to System IMO.
Whilst this is doable (i.e. using elevation tools to merge a .REG file or change permissions of the registry key, as two examples), it adds a degree of complexity. I suggest continuing to use CUE as it appears to do what you want.
Nothing like making a user preference that affects their eyesight so easy to change or automate, eh, Microsoft?
For me, that is a huge clue. This tells us that as far as Windows is concerned, the brightness remains at minimum and has not changed. That makes me think that maybe one of these things is going on:
1) There is some issue with the driver.
2) Some systems have two methods that they use to adjust the brightness apart from the Windows settings. I have an older laptop where the brightness adapts automatically based upon the content of the screen. For example, a screen with a lot of white gets a very different brightness level then does a screen with a mostly black. In addition, there is a sensor built into the screen that works with the display driver to adjust brightness based upon ambient light.
The adjustment for both is in the display driver (in my case it's the Intel Graphics control software). It's been a long time since I played with that, but as I recall, one of those settings was rather obvious, while the other was not intuitively obvious. I think it had something to do with power saving settings. In any case, I turned both off as they were extraordinarily annoying.
In the Intel HD Graphics Control Panel > Power, despite the settings being different for Plugged In and On Battery The On Battery > Display Power Saving Technology setting needs to be Disabled.
In the Profiles section tick all the setting boxes, and Select Trigger > None > Apply > save the profile when done.
Windows 10
Manufacturers like Dell will also put in Power Plans, whatever Active plan you are using check out the Advanced Settings, probably the Intel graphics setting or Display settings. Not necessarily Brightness but Power saving measures as well.
Power Options > choose the active plan > Change plan settings > Change advanced power settings > ...
Look in drop down list for Intel(R) graphics settings, Display sections,
Over the years what appears in the Power Plans has varied, there used to be one for Adaptive brightness which has gone in recent years, good thing to as it was troublesome.
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