When converting a pptx file with an screenshot of a excel table the resulting table in PDF after the conversion is black. However if the actual table is include in the slide, it does get rendered properly and the resulting PDF contains the actual table.
I have observed the issue shared by you and request you to please try using latest Aspose.Slides for .NET 20.2 on your end. In case there is still an issue then please share the working sample code along with source file and generated output with us.
I have tested in aspose 20.1 and it has been resolved then thanks.