It is not an issue of trial version limitation. We have tested the scenario in our environment using following code snippet with Aspose.PDF for .NET 20.5 and were unable to notice any issue.
Aspose.Pdf.Document pdfDocument = new Aspose.Pdf.Document(dataDir + "Large doc.pdf");
HtmlSaveOptions options = new HtmlSaveOptions();
options.PartsEmbeddingMode = HtmlSaveOptions.PartsEmbeddingModes.EmbedAllIntoHtml;
options.RasterImagesSavingMode = HtmlSaveOptions.RasterImagesSavingModes.AsEmbeddedPartsOfPngPageBackground;
options.LettersPositioningMethod = HtmlSaveOptions.LettersPositioningMethods.UseEmUnitsAndCompensationOfRoundingErrorsInCss;
options.BatchSize = 640;
pdfDocument.Save(dataDir + "output20.5.html", options);
Would you please make sure to use latest version and in case you still face any issue, please share the code snippet that you are using. We will test the scenario in our environment and address it accordingly.