I have been unable to successfully save any Presentation to a memorystream without being prompted to repair the document when opening in Powerpoint 2019. Please refer to the below code snippet to see if you can reproduce your own side.
Using p As Syncfusion.Presentation.IPresentation = Presentation.Create
Using ms As New System.IO.MemoryStream
ms.Position = 0
Using fs As New System.IO.FileStream("E:\testiems.pptx", System.IO.FileMode.Create)
fs.Write(ms.GetBuffer(), 0, ms.GetBuffer().Length)