Summary

Using the Update Custom Field post function to append a user to a multi-user custom field will fail if any value is entered on the field

Environment

Jira Cloud

Steps to Reproduce

1. Create a custom field of the kind User Picker (multiple users)
2. Add a post-function to a workflow transition of the kind Update Custom Field to update the field and select the option to append the value

Expected Results

The selected value will be appended to the field value

Actual Results

When trying to execute the transition the post-function will fail if a value already exists on the field