Google Chrome 67.0.3383.0 (Official Build) dev (64-bit)
Firmware Version Google_Samus.6300.276.0

Steps to repro:

# Attach external monitor. For this specific Samus device, I used a display port to USB C adapter and a display port to display port cable to an HP monitor. 
# In Displays settings, choose Extend
# Take a screenshot using ctrl + switch apps button
# Note that two downloads are made - Display 1 and Display 2 
Expected: both the local and external monitors are included in screenshot preview indicating that both have been captured
Actual: only the local screen is in the screenshot preview

This led me to believe that only the local screen was included in the screenshot. Only after going through my downloads later did I notice that both screens had been captured as two different image files.