Often it is necessary to keep parent issues and sub-tasks in sync. To do so, the following automation rule comes with a handy configuration that automatically updates specific sub-tasks' field values with their corresponding parent issue's value.
In our use case we want to automatically update the priority field.
Field changed event
Create a new rule and name it appropriately.
Providing a description will help you to identify what the rule does but this step is optional.
Add a Trigger → Field Changed Event
Choose Any value
Add Selector → Issue selector → Sub-tasks
No further configuration needed.
Update field configration
Next to Issue selector click on Add → Action → Update Field
Choose Value returned by parsed expression
This is how the configuration above should look on your screen