While using the latest version of Aspose.Words i.e. 13.8.0, I managed to reproduce this issue on my side. I have logged this issue in our bug tracking system. The ID of this issue is WORDSNET-8878. Your request has also been linked to this issue and you will be notified as soon as it is resolved. Sorry for the inconvenience.
Moreover, as a temporary workaround you may use the following code:
Document doc = new Document(@"C:\Temp\input.docx");
PdfSaveOptions saveOptions = new PdfSaveOptions();
saveOptions.MetafileRenderingOptions.RenderingMode = MetafileRenderingMode.Bitmap;
doc.Save(@"C:\Temp\out.pdf", saveOptions);
In this case, Aspose.Words invokes GDI+ to render a metafile to a bitmap and then saves the bitmap to the output document.