Thank you for posting.
I have observed your requirements and worked with the presentation file shared by you. I have been able to reproduce the issue. A ticket with ID SLIDESNET-36878 has been logged 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.
Also, I would like to add that POT format is not supported by Aspose.Slides however POTX format is supported. This issue has been logged into our issue management system for the resolution of issue with ppt file format
We are sorry for your inconvenience,