Jira does not fire any issue event when some of the fields are updated. The following fields won't get an issue update event:
- Epic link ( for more details please refer to the Atlassian ticket)
Changing the issue link does not fire an issue event in Jira lower than 7.0.5.
JIRA on-premise does not fire an issue event on an issue link update in versions lower than the 7.0.5 version.
You should update any other field on the issue to fire an issue event. It will start the synchronization.