I have worked with Aspose.Slides for .NET 5.2.0 and unfortunately I have not been able to reproduce the issue specified. It will be really convenient if you may please share the code snippet for necessary investigation. I have created an issue with ID 27444 as investigation in our issue tracking system to investigate the issue. This thread has also been linked with the issue so that you may be automatically notified once the issue is resolved.
We are sorry for your inconvenience,
I have worked with the presentation file shared by you and have been able to observe the issue shared by you. I have also observed the exception while generating the slide thumbnail. The issue description for 27444 has also been modified. We will share the further information with you once it is shared by our development team.
Thanks and Regards,