You should be able to save that setting to a profile. That profile should be kept. I know of 2 instances that will default back. One is when you update a driver it will default and two is when the driver crashes it will go back to default. Just reload the profile you saved.
If Windows is not retaining this new setting it may be because it is still referencing old information on startup. Disable "Hibernation" and "Fast Startup" in Windows 10.
When you are in Wattman I would suggest you slide that Power Limit slider over to the right to it's maximum also (+50). That will likely keep you from having to come back with some other common issues.
Good Luck and Congratulations on the new card, I have the same one! Enjoy that monitor too!