Editing a subtask removes its sprint value

As reported at https://confluence.atlassian.com/display/AGILE/JIRA+Agile+6.3.9.2+Release+Notes?focusedCommentId=550109398#comment-550109398

We're seeing something else weird on subtasks, that could be related to the display of the sprint field within the edit issue dialog.

For example we have an issue with a couple of subtasks scheduled in sprint. If you go to the edit screen for one of these subtasks and click update (regardless of whether or not you change any values as part of the edit), the subtask's history (and the email notification) will tell you that the sprint value was emptied. (The subtasks issue detail screen still shows it as belonging to the correct sprint, and it still shows up on the scrum board's work area). We're on JIRA 6.2, and see the above with both agile 6.3.9.1 and 6.3.9.2 (we never downloaded 6.3.9).