Could you, please, be more specific about your hardware. i.e. 
1. Which chromebook do you use?
2. Which monitor do you use? Did you test it with other monitors?
3. Which usbc->displayport dongle do you use?

Thank you!

1. HP Chromebook 13
Dual-core Intel m7-6Y75;  1.2GHz, 4MB
16GB 1866MHz DDR3 Memory
32GB SSD
Intel HD Graphics
13.3-inch 3200x1800 LED-backlit display

2. HP Z30i. I haven't tested it with other monitors, but I have attached other laptops/desktops to this monitor without issue.
3. Google USB-C -> Displayport labelled as
"Designed by Google  Model No. 6691-10MU-0183"

I've also used a usb-b -> mini displayport -> display port that I no longer have in my possesion, and it fails to work in the same way.

Thank you for details! Unfortunately, I can't reproduce this bug. I tried with two different monitors and several usbc->dp dongles using the same version of Chrome OS - in all cases chromebook was extending to an external monitor after suspended mode properly. 

So, could you, please, try with another monitor and, if it's possible, with another HP Chromebook 13?

Thank you!

I am not surprised you were unable to reproduce this issue since it can take days or weeks of usage for it to manifest. I'm not sure if it's a fluke of the details with the awake/suspend cycle, or somehow related to daily detach/reattach of the laptop to the monitor. As such, I'm not sure that I can devote more time to trying to debug the issue. As anecdotal evidence, local tech support says they've seen this issue with the HP chromebook before (and provided the suggestion of powerwashing to fix); maybe the HP chromebook is the cause of the issue.

Thanks,
Tony

I tried about 30 suspend/wakeup cycles, but everything was ok. I just want to be sure I'm doing everything right. May, the problem is in your monitor or in your chromebook. What is estimated appearance probability for this bug? 25%/10%/1%?

Thank you!

Very low likelihood, 1% or less. The problem is that the condition is sticky, and once it occurs the machine generally won't extend the desktop again. I highly doubt the problem is the monitor, since I've been using it with this and other machines for years w/o issue. It could possibly be the chromebook, I may try to get it swapped for a different machine.

Thank you! It's very difficult to investigate something like this without logs. Moreover, I can't reproduce it even with 1% rate and we can't be sure this bug even relates to chromeOS. As I said, the problem could be in monitor/cable/usbc dongle/etc.