I have worked with the DWG file shared by you and have been able to observe the issue specified. An issue with ID CADNET-734 has been created in our issue tracking system to further investigate and resolve the issue. This thread has been linked with the issue so that you may be notified once the issue will be fixed.
These values are stored in file (available as Header properties in cadImage) and we retrieve them properly. We internally transform them to zeros correctly because Model sheet is empty (by the way, such values in file may occur not only because of empty sheet). If you want to get size of each sheet, you may use:
Layout Model has size 0x0
Layout Sheet Form D has size 34,5x23
Please note that previous version Aspose.CAD for .NET 18.8 returned size of the layout instead of Model size. I hope the shared elaboration will be helpful.