My current setup is my Dell XPS 9560 (1080p) with 2x 1440p 75hz monitors. Everything works fine on first boot: I turn on the laptop, type in my password, shut the lid and just use the two external monitors.
However, if I put the computer to sleep and then try and wake it up again, only one of the two external monitors comes on. Sometimes both of them do but that only seems to happen around 1/10 times.
The monitor appears to wake up (it turns on) but it flicks between the sources and reports "no signal".
I have the laptop running on the integrated Intel graphics but the same thing appears to happen on the Nvidia graphics as well.
The easiest way to get the monitor back on is just to restart the entire machine and 99% of the time it comes back on. Either that or if I fiddle around with settings long enough and re-plug the HDMI cable a few times it sometimes comes back on.
I am running Ubuntu 18.04 LTS on the machine and this is the only problem I have had so far!
EDIT: The current workaround I am using to get the monitor to come back on is to change the refresh rate from
59.95hz and then back again. This pretty consistently works but obviously isn't an ideal solution.