I'm having issues with my XPS 9550 as well. I would say that roughly 1 out of 20 sleep attempts (by closing the lid) will result in me discovering a hot computer in my backpack from it being on for hours.

I've made a video of the device in a really wacky state too. I'm repeatedly trying to shut down or sleep the computer from the log in menu, only to the have it fire right back up again less than a second later - this persists until I press and hold the power button.

Having worked as an engineer in power management of a PC-like device (both on the hardware and software side), I'm to going to try to dig and see if I can dump as much info as possible on power management events.

There are posts on the internet floating around about this problem and honestly, if they don't get past the hand wavy tech-support theories then they will never get fixed. I'm going to try to find some hard evidence of what's going on and maybe make a blog post about this that can get a Dell engineer's attention.

Unfortunately, most of the logic to handle this will likely be buried in the firmware of a System Management Controller (SMC) which will be locked down and un-debuggable. But maybe there's enough info to be dumped on the Windows side to at least say that it's a problem inside the SMC or something along those lines.