we are using the below sample code to generate a thumbnail image for a pptx file.
Thanks for inquiring Aspose.Slides.
I have worked with the presentation file shared by you and have been able to observe the issue specified in generated thumbnail for set zoom level. An issue with ID SLIDESNET-35374 has been created in our issue tracking system to further investigate and resolve the issue. This thread has been linked with the issue so that you may be automatically notified once the issue will be resolved.
For the time being, I suggest you to please use the following statement to serve the purpose.
image = oSlide.GetThumbnail(1.0F, 1.0F);
We are sorry for your inconvenience,