Issue results contain a good deal of contextual references: linked issues, sub-tasks, project, etc. However they do not contain a reference to the parent issue.

https://developer.atlassian.com/cloud/jira/platform/rest/v2/#api-rest-api-2-issue-issueIdOrKey-get

This omission makes it difficult to construct tree-like views on issue queries. The backing data structure clearly maintains this information, because issue queries can filter by parent ID. However, it is not possible to disambiguate the parents of multiple issues returned by the same JQL query.

The message boards are littered with requests for how to obtain the parent of an issue, with the answer invariably being run a secondary query. The limitation can be seen in Jira's own UI as the issue view slowly builds the branch structure, one parent query at a time. All of these use cases would be made much simpler and reduce API spam by adding a reference to the parent issue.