Getting the parents of subtasks using JQL is not supported out of the box. You must use a plugin such as Power Scripts or Scriptrunner using Jira Server or Data Center. Here is a JQL query I tested using the parentsOf() routine using Power Scripts.
key in parentsOf("issuetype = Sub-task AND status != Done") AND status = Done
For more information see this documentation on parentsOf().
Because of Confluence Cloud security features, you may need to open the documentation in incognito mode or by logging out of Confluence.
Full disclosure, I am a product engineer for Power Scripts and work for Anova Apps, an Appfire company.
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