I have come to observe that the Navigate to Activity does not work reliably in the Unattended mode. I test the Automation in Studio and it works. But when triggered as an Unattended Automation from Assistant, the Navigate To activity fails to recognize the parent window 2 out of 4 times!
I am using the Edge Browser to run the automation.
I am not sure what is the approach to debug this. Here is the package stack and how this activity has been configured.
In the second picture, the Browser App reference is passed to the flow from a previous flow that uses the Open Browser activity to login to the Web application and return the Browser app reference.