I have the same issue with Dell U2913WM. I recently purchased a new Apple iMac 27" (2017) and had to buy a special USB-C/TB3 to DP cable so I can use a U2913WM as a 2nd screen. If I power down the Dell monitor by pressing its power button, it will not turn back on (no lights) unless the DisplayPort lead is detached or the power source on the other end (eg. iMac) is turned off.
I have to ensure the Dell monitor is powered on first before the iMac is turned on. If not, I have to disconnect the lead. It is very annoying. I actually have a 2nd Dell U2913WM I am selling, and I tested it on the same setup and the same thing happens, meaning its all the Dell U2913WM monitors. Only happens with DisplayPort.
What could be causing this? Some kind of power draw from the other end (eg. iMac)?
The U2913WM mDP to DP cable will not suffice for the new Apple USB-C/TB3 port. Which is why you had to buy the special USB-C/TB3 adapter cable.
As you can see in the image below taken directly from the November 2012 U2913WM page,
the Apple proprietary hardware, operating system, and drivers were not tested/validated by Dell. You would best suited to ask the Apple users on their Forum about the power issues when using their hardware on our monitor. Ask them if they know of a USB-C/TB3 to HDMI adapter cable.