Have you looked at any of the BIOS settings, specifically Block Sleep and Wake On Lan? This support page has more information.

Hi,

I faced a same problem. The BIOS settings, Block Sleep and Wake On Lan both disabled.

Any updates?  I'm having the same problem with a bunch of 5400 and 5500s.  vPro won't even work when they do this.

Thanks

I am having the same issue. I am trying to configure machines at a remote location and I have to keep getting someone onsite to wake up the machines. 
They are plugged in and set to not sleep on AC power, and connected via LAN.
I have had success in disabling the EEE (energy efficient ethernet) setting in the NIC configuration, but it seems to be happening again.
Dell Latitude 5400. 

Has anyone figured this out yet?  We've deployed quite a few of these now and they all have this issue.  In some cases the laptop will no longer ping just after about 30 minutes, or it randomly responds, if no one has logged on to it.  If the network cable is unplugged and reconnected the connection stays normal again for a while, but then stops again.  Also, this is occurring either with the network cable directly connected to the laptop or while connected to the WD19 dock.

Looks like our shipment of laptops are also exhibiting this behavior. Will report back if we find any leads!

Same here. Our IT department says this is a "new feature". When I hit the power button the fan stops momentarily, but then resumes. The light on the power button turns off (IT says it is to conserve power, but how much power does one LED really consume?), but if I touch the keyboard or move my external mouse the screen immediately comes back on. If I pick up my mouse to turn the power button off, then the laptop comes back on and I have to put it to "sleep" again.

So annoying because there's times I've put it in my backpack and hours later I pull it out and the fan is running because it got hot and now the battery is almost dead. Also when it was new, it would not recognize any headphones plugged in after it was already powered up. IT finally fixed that issue.

I wish Dell would fix this "new power feature". Really wish I had my old 7480 back.

We also have the same issue with all of the Latitude 5400's we purchased and it would drive me crazy because it would prevent us from accessing the laptops remotely. I believe I have figured out what is causing it however. Even with sleep disabled, Windows 10 has a feature called "Connected Standby". It's only supported if the device itself supports it (which the Latitude 5400 does). This mode basically replaces normal sleep mode altogether. What Connected Standby does is it puts the computers into a low power state but still allows it to be waken up for any Windows Updates that need to be installed. We used Lenovo computers prior to switching to Dell and I noticed that the Lenovo's we bought did not have this feature which is why we never experienced it until switching over to Dell.

In order to disable this, open up regedit and go to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Power and the set the key "CsEnabled" to 0 and then reboot. This will disable connected standby mode and enable normal sleep mode on the laptop itself. Since the normal sleep mode is now enabled, you can control if the computer goes to sleep as you normally would through the Windows Power settings in Control Panel. I hope this helps.

I can confirm that this setting keeps the laptop from going into the sleep mode described by the OP.  The laptop will now "wake up" by touching the trackpad, instead of having to press a key on the keyboard or pressing the power button again.  However, I'm still having the issue where the laptop is randomly failing to respond on the network after a period of time.  For me it's about 30 minutes after power up.  Using the latest BIOS and latest drivers.  Unable to RDP to the laptop, on the same subnet, due to the connection timing out. 

The trick that seems to have been working on one machine was Disabling the "C-states" located in the BIOS. This was suggested in another thread on Dell forums. We did also make sure that sleep states for Windows was disabled and that the green low-power modes for the ethernet was also disabled.

We've had one computer that we noticed was having issues dropping network connections and 'resume' once keyboard or mouse was moved. After disabling "C-States", things seem to work as expected.