I have observed the images shared by you. I have only been able to spot the difference as the text, in thumbnail which is generated by Aspose.Slides, appears bigger and bold. Is this what you mean? Please let us know if you are observing any other difference, so that I may log a ticket in our issue management system with all the details in order to investigate and resolve this issue. Also, I request you to please try using Aspose.Slides for .NET 15.6.0 on your end before you share your valuable feedback with us.
I have worked with the presentation file shared by you but I am unable to reproduce the issue on my end. I have attached the thumbnail which I have created on my end using Aspose.Slides for .NET 15.6.0. The text is not appearing as bold or bigger. However, I can see some additional spaces between characters which is the only issue appearing on my end. Please share your environment details so that I may reproduce the issue on my end.
The issue that the text is appearing bold was resolved using Aspose.Slides for .Net 15.6.0. But the issue that the text is not applied correct font is not resolved yet.
It looks like some additional spaces between characters, because the font is different.
The correct font is "MS PGothic". It is called a proportional font that the character width is different font for each character. The wrong font is "MS Gothic". It is called a monospaced font that the character width is same font for each character. Especially full width parentheses is different. It used "MS PGothic" are about 0.5em, it used "MS Gothic" are 1em.
The powerpoint file is setted the major (Headings) and minor (Body) fonts are "MS PGothic".
I set the text to the major font using Microsoft Powerpoint, the text was applied "MS Gothic" in thumbnail using Aspose.Slides. I guess Aspose.Slide can not get the font in the portion and "MS Gothic" that is the default japanese font is applied.
I have bad English, so I apologize if I say something strange.
I have observed your comments and I have been able to reproduce the issue. A ticket with ID SLIDESNET-36646 has been logged in our issue tracking system to further investigate and resolve the issue. We will take into consideration all your findings. This thread has been linked with the issue so that you may be automatically notified once the issue will be resolved.