When using Slack Workflow Builder to create a Jira issue via the Jira Cloud for Slack app, dynamic person variables (e.g. "Person who submitted this workflow") cannot be used to populate user-picker custom fields. The step fails with:

 {"errors":{"customfield_XXXXX":"Specify a valid value for customfield_XXXXX"}}

The app passes the Slack user's display name as a plain string, but Jira's API requires an accountId. The app should resolve the Slack user to a Jira accountId automatically (via email match) before submitting the field value.
Workarounds today: hardcode a fixed accountId, or use a proxy text field + Jira Automation rule to set the field after issue creation.