QueryDB brings the data into a 2DS. You can display that in the form (the field would show the data from the 2DS). If you want the data stored in the main DOM and promoted to a column in SharePoint, the value needs to be set.
Without seeing the form template I wouldn't be able to diagnose the problem, but it sounds like the field in the main data source is not receiving the value. What you can do is, before submitting, save the XML locally and open it to check the value stored in the troublesome field. If it doesn't have the value, then you know there's a problem in the transfer of the data from the QueryDB 2DS to the main data source. If it does have the value, then maybe there's a problem in the promotion of the property.
Anyway, hope this helps, and if you're still stuck please attach the xsn (as mentioned in the post above mine). Thanks.