I would take a look at the Communities post When you create a sub-task can you automatically inherit certain fields from the parent for the way others have resolved this issue in their instances:
Here is the advice from one user:
"In the create issue transition you can set the "Set Field Value from Parent" option in the post-functions. You can either do this in the default workflow you are using for the project, or make a separate workflow for sub-tasks. When only using this post-function, changing the value of the field in either the parent or sub-task will have no influence of the others value."
In addition you can also try Automation for JIRA.
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