Symptoms

After upgrading to JIRA 6.0, the Update Issue Field post-function stops working in the Create Issue transition after upgrading to JIRA 6.

Diagnosis

This happens if your Create Issue transition is configured as below, where the Update Issue Field post-function is triggered after the Creates the issue originally post-function:
 

Cause

This happens due to a change in JIRA 6. In JIRA 5.2.11, this worked whether the Update Issue Field was before the Creates the issue originally or after. This change is currently being tracked as a bug on JRA-33182 .

Workaround

1. Edit the Workflow showing this behaviour.
2. Move the Update Issue Field post-function to the top of the list (before the Creates the issue originally):
    
3. Publish the Workflow Draft that was created.