I believe there is a bug in your Table implementation with regard to how borders are added. When the Width property of the BorderLeft/Top/Right/Bottom is set before the FillFormat.SolidFillColor.Color property, the result is a black border where the two cells intersect.
I’ve attached a screenshot, the actual powerpoint, and code to reproduce the problem. I’m using Aspose.Slides v16.8.0
Since there is a workaround, I don’t need a bug fix, but thought you should know. Thanks.
I have worked with the presentation shared by you using Aspose.Slides for .NET 16.8.0 and have been able to observe the issue. A ticket with ID SLIDESNET-37912 has been added in our issue tracking system to resolve the issue. This thread has been associated with the ticket so that we share notification with you once issue will be fixed.
We are sorry for your inconvenience,