Default value of variable is empty when the workflow is started by activity \Call web service\"

Hello everyone, could you please explain the behavior below?

There are two simple workflows:

1. Parent Site Workflow - it starts another workflow "Child Site Workflow" via activity "Call web service".
2. Child Site Workflow - it logs in history the value of variable "TestDefaultValueOfVariable". This variable has a default value.

The expected result is default value in history, but it is empty!

But if "Child Site Workflow" is started manually or via activity "Start workflow" then default value of variable is present in history.

It is unexpected for me and looks like a bug.

Thank you in advance for your answers.