• Currently, {{_parsedData}}is an internal object in the flow. Only {{_payload{}}}, {}{{_headers{}}}, and {{_url}} are supported as dynamic variables.
• Suggestion to make use Jira custom field from _parsedData._customFields in Ops/Incident sync custom properties

Possible workaround :

Workaround 1:

Using Jira Automation, trigger on changes to the custom field and call the Create Alert REST API directly, bypassing Sync altogether. You can do this by:

• Configuring an Automation rule with a field change trigger for custom field, and

• Using a Webhook action in that rule to call the Create Alert endpoint from the Jira Service Management Ops REST API: The Jira Service Management ops REST API

Workaround 2:

Using combination of Jira Automation + Sync:

• In Jira Automation, use Field Value Updated Trigger -> Copy the custom field value as label to the work item
• In the Ops side, under Sync conditions -> Use Label property -> "equals" -> "<Label_value>"
• Use to create alert for the required labels. 

This way, you can fully control how the custom_field value is passed into the alert payload, without relying on Sync’s current limitations around _parsedData.