Yes, it could be because you are using Aspose.Words in evaluation mode. The evaluation (trial) version of Aspose.Words (without a license specified) provides full product functionality, but it inserts an evaluation watermark at the top of the document on open and save, and limits the maximum document size to several hundred paragraphs.
If you want to test Aspose.Words without the evaluation version limitations, you can also request a 30-day Temporary License. Please refer to How to get a Temporary License?
In case the problem still remains, please ZIP and upload your input Word document you are getting this problem with here for testing. We will investigate the issue on our end and provide you more information.