As a new owner of the 38UC99 and a 15" model with the 460 graphics chip I've encountered very similar problems using the laptop in clamshell mode:
Having both the power cable and the display connected over USB-C there is a very high chance the machine will crash when trying to wake it from sleep. On wake the display will light up shortly and then notify me of no incoming signals. Then, when I open the laptop the display will remain black (sometimes with the backlight on). I can only resort to a hard reboot or leaving it awake for a couple of minutes, usually it will reboot itself yielding a kernel panic report and a gpuRestart report as a result. When this happens I also have to disconnect/connect the usb-c cable for a couple of times, because the laptop will regularly just stop detecting the display.
I've contacted Apple care and tried the following:
- Test sleep/wake behaviour without an external display connected
- Test sleep/wake behaviour with the external display connected in clamshell mode while booted up in safe-mode
In both scenario's sleep / wake works without error, the behaviour during the wake process is also much more stable independent of the amount of time spent sleeping. I've read somewhere that during safe-mode a generic graphics driver is loaded which points at faulty AMD drivers in mac os sierra.
I've also tested the following scenario:
- Disable automatic graphics switching
- Enable sleep prevention when the display is turned off
- Only put the display to sleep completely circumventing mac os sleep (the machine always stays on)
Problems occur less frequently but even in this scenario I've found that the machine sometimes errors out when having to wake the display with similar symptoms as described above.
I am currently awaiting word from Apple care, keep you posted.