Based upon the explanation in this documentation, completion is computed/rolled up for the issues in the plan and is not in one field. There also appear to be some settings impacting the behavior.
You could probably re-compute the value using automation rules, JQL scripting add-ons, or by pulling the data (export or REST API).
Thanks for detailed response. I was able to extract "Status category change date" field in API call. It actually populates the date when issue has moved from To Do to In Progress, In Progress to In QA etc.
What is not clear is how to use this field to calculate % Completion.
Could you elaborate a bit more on this?
The Status Category Change Date is set when an issue moves between the status category values of To Do, In Progress, and Done. Each status in your workflow is categorized as one of those, and so Status Category can help summarize the workflow.
Please note that Status Category Change Date is a text field, so you would need to convert it to a date when using it. For example:
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