Unable to update issue via API if linked to an Epic

Reproduction:

1. Create Epics: EP-1 .. EP-11
2. Associate a test ticket TST-1 to EP-1.
3. Update TST-1 to change the assignee for example:

curl -k -D- -u user:xxxx -X PUT --data "{ \"fields\": { \"assignee\":{\"name\":\"admin\"} } }" -H "Content-Type: application/json" https://jira/rest/api/2/issue/TST-1

4. Error received:

{"errorMessages":[],"errors":{"customfield_11270":"Epic with name 'EP-1' is not unique."}

The GreenHopper Epic Name field seems to be validating incorrectly when searching for duplicate Epics. It's matching EP-11. If I delete EP-11, it works.

See also : https://confluence.atlassian.com/pages/viewpage.action?pageId=369295497

This bug prevents us from automating many tasks in JIRA.