When you open an issue with links or sub-tasks, you see them in a list below the description/attachments section, but when you open the sub-task, the only way to see the parent issue is in the "path" shown above the summary. 

This is not a very clear way to show the parent link and this suggestion is to have Jira show the parent issue in the sub-task in a similar way it's shown the other way around. Show the parent issue below the attachments section in the issue view with the summary, status, and other fields.