It works for summary, description and compontens, but it don't inherit the assignee.
This is the script I used as Server-Side-Script for an added visible field (summary):

Still, when I click on "Create a subtask" Assignee-Field stays empty.
When I add this line

the assignee appears in the description-field. So it is possible to get the value. Why it then won't work with the assignee-field?
We use Jira Server 7.7.0 and ScriptRunner 5.3.13 (latest Version).
We update Jira tomorrow to 7.9.0