There is a fundamental differences between Jira Server/Data Center and Jira Cloud: In Jira Cloud post functions of apps are executed asynchronous as a 'background job'.

• This means a post function will be executed after the transition has completed on Jira Cloud.
• There is no guaranteed order in which several post functions are performed. They might be executed in a different order than the one you had set them up in Jira's workflow configuration. So you cannot rely on any result of a 'previous' post function. The order how the post functions are executed might be different every time the transition is performed.
• When a post function has a problem (due to misconfiguration or the current data of a Jira issue), the transition (and all other post functions) will be performed nevertheless.
• The result of a post function might not be visible immediately in the browser.
• There are no error messages displayed to the user.