Greetings from syncfusion.
We have checked with the issue which you have mentioned. Kindly use the “PdfLoadedStyledAnnotation” class to retrieve the modified date of existing annotation. Please refer the below code snippet for more details,
//Get the annotation collection
PdfLoadedAnnotationCollection collection = loadedDocument.Pages.Annotations;
//Get the annotation
PdfLoadedStyledAnnotation annotation = collection as PdfLoadedStyledAnnotation;
//Get modified date
DateTime modifiedDate = annotation.ModifiedDate;
Please try the above solution in your end and let us know if it solves the issue.