The content controls ID's must be set automatically when once clones or inserts an SDT.
Workaround: Manually set the ID
SdtRangeStart start = grpContentControls.Where(c => Convert.ToString(c.SdtProperties.Tag) == "purchlastname").First();
var properties = new SdtProperties(start.SdtProperties);
properties.ID = 123456;
var currentItem = editor.InsertStructuredDocumentTag(properties);