@Bill Sheboy tagging you as you recently helped me with Solved: Jira automation - Parent Issue with open sub-tasks... (atlassian.com) and this is a related issue 
Please refer workflow screenshot below, so basically this time - when I mark the parent issue as Done with resolution as either Won't Do or Duplicate and there are child issues in not Done status then it will move them to Done with resolution as Won't Do.
I have Epic A with child issues Story X & Story Y. Story X has 2 open sub-tasks and same for Y. When I am marking Epic A as Done with resolution Won't Do the rule is also transitioning the 2 linked Stories to Done with resolution Won't Do - which is fine and what I expect.
However, the open sub-tasks linked to the 2 stories stay in their original status, what do I need to add to the workflow please so that those get updated as Done as well with resolution Won't Do?
