If PPT is parsed through asposed -20.10-JDk16.jar, it can only be parsed into general HTML, and the animation in the original PPT cannot be parsed into HTML
You are right as at the moment the support for preserving animations in exported HTML not available in API. A ticket with ID SLIDESJAVA-35760 has already been created to provide the support. I have linked this thread with issue so that you may be notified once the support will be available.