In Jira Cloud, when When a custom field is added to a workflow transition screen in Jira Cloud, the value is shown as text and not a fixed drop downdropdown. This behavior behaviour also occurs when creating or editing a subtask.
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In Jira Server and Jira Data Center, this behaviour is not seen and you can change your selection as intended.
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Refer to this Atlassian ticket to learn more: https://ecosystem.atlassian.net/browse/FRGE-576.