This is not true. When tags are subscribed to OPC items, the OPC server makes a best effort to deliver the data at the pace indicated in the scan class, but makes no guarantee. No guarantees for order of delivery, either. Tag change events can and do start executing right away, while other tags on the scan class are still updating, if updating at all at that time.
In your case, I would have a tag subscribed to the PLC trigger item as you describe, with tag change script, but I would use system.opc.read*() calls to retrieve the rest of the data. That's the only way to be sure the rest of the data is post-trigger. This is precisely what "OPC Read" mode does in transaction groups, fwiw.