Scriptrunner post function in workflow transition doesn't show custom field

[Joeri Gydé]

I created a multiline custom field and associated it with a create/view/edit screen of all issuetypes in my project.

I have a transition in a workflow with a post function triggering a scriptrunner script. The user is set to Initiating User. In that script I do following Unirest API call:

def issue = Unirest.get("/rest/api/latest/issue/${issueKey}?expand=customfield_12549")
        .header('Content-Type', 'application/json')
        .asObject(Map)
        .body

Assuming the expand keyword is optional.

The variable issue contains a map to the asked issue.  However the concerned costum field is missing in the data.

Strangely enough making the same API call in the Scriptrunner Console gives a data structure containing the wanted custom field.

What makes a possible difference between both calling situations?