We were able to reproduce the issue in our environment while using Aspose.Diagram for .NET 19.7 with following code snippet:
Aspose.Diagram.Diagram theDiagram = new Aspose.Diagram.Diagram(dataDir + "TheFileNameIsRenderedInTheHtml.vsdx");
Aspose.Diagram.Saving.HTMLSaveOptions options = new Diagram.Saving.HTMLSaveOptions();
theDiagram.Save(dataDir + "TheFileNameIsRenderedInTheHtml.html", Aspose.Diagram.SaveFileFormat.HTML);
We have logged it as DIAGRAMNET-51676 in our issue tracking system. We will further look into details of the issue and keep you posted with the status of its correction. Please be patient and spare us little time.
We are sorry for the inconvenience.