A second case (please see screenshots)
I have worked with presentation file shared by you using Aspose.Slides for Java 16.2.0 on my end in Windows 7 with JDK 1.7. I have not observed any issue in generated thumbnail. For your kind reference, the generated thumbnail is shared. Also, in your presentation, Arial font has been used. Can you please make sure that if Arial font is accessible in your Operating System. Can you please also share the Java and operating system details along with used sample code so that we may investigate the issue further to help you out.