Currently when running tests against a private test agent instance on Chrome with mobile emulation, the received screenshots are of a minimized browser window:
This behavior appears only when mobile emulation is enabled on Chrome browser.
 latest wptdriver installed
 Windows 7 VM
[*] VM is restarted and no active RDP is done afterwards as per requirement
What is more interesting is that when I connect the VM via RDP, if I’m watching the test execution, the problem disappears.