I use my desktop system for my recording studio.  I have High Performance set and never sleep and I haven't had any problems with it reverting to what yours is doing.  

I have had some power settings change after a Windows update, such as the Fall Creator's Upgrade.  I am in the "Windows 10 Insider" program and get advanced Win 10 versions (dual boot on a separate drive with my production system).  Every time I get a new version I have to update my power plan. 

You could Create your own power plan (using the High Performance as the base) and see if that helps.

Here are some screenshots of my Power plan and the additional panels that allow access to create your own power plan.

Hmm.. A WIn10 patch was just rolled out recently.  

But I'm pretty sure that the power settings have reset on me at least twice since then.  Is there any way to lock down the setting?  Perhaps within the registry?

I know of no way other than possibly creating your own power plan as suggested.

I do support on other forums and don't recall seeing this problem posted, yours is the first I've seen.

The forum is going down (to read only) for forum software update (change to a different forum software) on 1/5.  This is probably the last I can respond until the forum comes back up (eta 1/10).

I don't think its a "Dell PC" problem, more some software.  

You may want to consider pursuing this on the tenforums.com and see if someone there can offer anything. 

Hi Mele20

i have exactly the same power setting problem as yours. My specs is the same as yours.  Please do share if you manage to find a solution. I have logged a support case to Dell some 2 weeks back but never hear a word from them. 

Device specifications
XPS 8930
Intel(R) Core(TM) i5-8400 CPU @ 2.80GHz 2.81GHz
Installed RAM
8.00 GB (7.81 GB usable)

The power plan reverting to Power save may be due to Intel's Ready Mode Technology (RMT).

Now there’s no waiting to resume activity on your PC. With Intel Ready Mode Technology, your PC resumes to fully functional in an instant. It works by providing an alternative to traditional sleep—a quiet, low power state similar to the kind used by your cell phone and tablet.

The FAQ says:

Does Intel Ready Mode Technology change my OS power plan?
Intel Ready Mode Technology temporarily changes the power plan to power saver mode when your computer transitions to the Intel® Ready Mode Technology state. On resume, your power plan is restored.

https://www.intel.com/content/www/us/en/support/articles/000020651/technologies.html

It may be changing your power plan to Power saver but not restoring it on resume. Maybe a bug or a conflict with something else.

Check to see if it's enabled and if so, disable it and reboot. Here's a link to the Setup and Configuration Guide for Windows 7 and 8.1. I didn't see one for Windows 10, but it's probably similar to the one for 8.1.

https://www.intel.com/content/www/us/en/support/articles/000006274/technologies.html

The above poster Mele20 is currently testing this.

I also have the same problem. Every night between 8 and 1 am est it seems to reset. 

Specs 

XPS 8930

i7 8700

GTX 1080

I have heard back from Mele20 that disabling Intel RMT worked for her. Mele's a smart cookie and had captured a number of Intel sleep/wake errors in Event Viewer at the times the screen would not wake up. I analyzed one of them and it pointed to the source as the Intel RMT NeverSleep Agent Service. That is what led to my suggestion to disable Intel RMT, since Intel RMT temporarily changes the power plan to Power saver mode and that is exactly what Mele20 was experiencing. During troubleshooting, Mele also created a custom power plan, which may or may not be needed.

So if you have the problem where your power plan is reverting to Power saver mode, try disabling Intel RMT and rebooting. See the second link in my earlier post for how to do this.

Of course, since there are multiple causes for power settings issues, this may or may not work for you. But it's one possible solution that I haven't seen discussed elsewhere.

I took delivery of a Dell XPS 8930 two days ago and have exactly the same problem. At random times the powerplan is changed from "Dell" to "power saver".  The Powersaver plan puts my PC to sleep which is totally unacceptable.  There are multiple error entries in the windows system log when this happens (Event ID 2000, task ID "Neversleep Agent Service", which mention something about SetPasswordRequiredValue, none of which I understand.

This would seem to be a Dell XPS problem.  It has never been reported for any other brand, and the problem seems to have started recently with Dell.

Further to my post above, Intel's Ready Mode Technology isNOT enabled. I didn't change that -- it's the way the PC was delivered.  So in my case anyway the suggestion in earlier posts about this possibly being the cause of the power problem is not relevant.

I've had an XPS 8930 for about 6 weeks now.  When I noticed the CPU wasn't preforming as expected, I changed the power plan from balanced to high performance and it made a huge difference with CPU intensive activities.  Since then the power plan has changed back to balanced by itself a dozen or more times.  With some help from the internet,  I found in the MS Event log where Intel(R) Ready Mode Technology\IRMTService.exe  is changing the power plan to balanced and it does not change it back.  Intel Ready Mode changes the power plan even though it says it's disabled. I'm tempted to remove it but am wondering why it was included disabled and, since it is actually doing things even though disabled, what the consequences of removing it would be.