Editing sprint puts issue in multiple sprints, only one shown in edit dialog

JIRA v6.4.12
JIRA Agile v6.7.11

We regularly move issues between sprints by editing the sprint field in the edit issue dialog, this is no longer working correctly with our recent JIRA upgrade to the aforementioned version.

Prerequisite: There are two sprints: November, December. Issue is in sprint November.

1. Edit issue
2. Delete sprint "November" from Sprint field
3. Type in new sprint name "December"
4. Click "Update"
5. Notice on the issue page under Active Sprints both "November" and "December" are listed
6. Notice the issue shows up in both "November" and "December" sprint boards
7. Edit the issue again
8. Notice only "December" is shown in the sprint field
9. Go to the Backlog page
10. Notice the issue is only listed in the "November" sprint

Another issue, Cannot remove an issue from sprint.
1. Edit issue
2. Delete sprint text
3. Click Update
4. Notice on the issue page under Active Sprints the original sprint is still listed