Issue Summary

When using workflow rules in team-managed project to update the labels field with certain values, it is supposed to append the field value with the existing value, however this does not happen. Whilst, the description states that it will add to existing values :

Steps to Reproduce

1. Create a team-managed project
2. Go to project settings -> Issue types -> click on "Edit workflow"
3. In one of the transitions -> under rules -> click on + icon 
4. Choose update field value -> choose the field "Labels" 
5. Add the desired labels
6. Save and close the workflow. 
7. Create an issue -> add a label of your choice -> transition the issue to the destined status -> you will see that the label value is replaced with the one entered in the workflow rule.

Expected Results

Values are appended to the existing values.

Actual Results

Workaround

Currently there is no known workaround for this behavior.