Sprint field for a sub-task does not hold the value of the sprint it is part of

Summary

When we start a sprint with stories containing sub-tasks, the sprint field value of the sub-task is empty. As a result, if a sub-task is converted to a story, it move out of the sprint, because the sprint value is empty.

Steps to Reproduce:
1. Start a sprint with issue containing sub-tasks.
2. Convert a sub-task to an issue.

Environment

JIRA 7.0.0, JIRA 7.3.6

Expected Result

The converted sub-task stays in the sprint

Actual Result

The converted sub-task moves out of the sprint.

Workaround

Manually add the task to the Sprint. 

• FYI, for active sprint, it'll trigger scope change.