When using the Project invoice (Excel) template, it is not possible to use LineProperty field from the underlying ProjTransEmpl record in a proper way. Which is in fact a limitation of the project invoice business document data model. So request to extend the model to be able to use this field (and probably also more fields) in the standard reporting model.
You can map the field, but at current it is only filled with the LineProperty value of the first ProjTransEmpl record on the invoice. So should be extended that per ProjTransEmpl record/line the field is updated/filled accordingly.