This seems to be a common problem with Windows 10. The culprit is Intel's Management Engine (IME) v11.* Ever since v10.* this has been a problem. What you need to do is to install a V9 or V10 IME and the system will operate as it should as it relates to power off, sleep, and hibernate. I had the same problem and regressed driver to v10 and everything has been fine since. 2 weeks and no troubles.
If you can't find the back level drivers on the Dell site, get the Intel ME driver here. Just use the driver install, and answer that you know you are going back level on the driver.