Hello,
I am not able to replicate this error with Aspose.Slides 2.9.2.0. Please see the code below; its source and output presentations are also attached.
Presentation pres = new Presentation(@"C:\source\source.ppt");
Slide sld = pres.GetSlideByPosition(1);
//Set the first shape text bold, underline and italic
Shape shp = sld.Shapes[0];
TextHolder thld = (TextHolder)shp.Placeholder;
Portion port = thld.Paragraphs[0].Portions[0];
port.FontBold = true;
port.FontUnderline = true;
port.FontItalic = true;
//Set the second shape text bold, underline and italic
shp = sld.Shapes[1];
TextFrame tf = shp.TextFrame;
port = tf.Paragraphs[0].Portions[0];
port.FontBold = true;
port.FontUnderline = true;
port.FontItalic = true;
//Write it on disk
pres.Write(@"C:\source\output.ppt");