When I save the XML file in iReport, all comments are deleted.
How can I include (or preserve) comments in JRXML files?
There is no way to preserve comments; you'd have to log a bug against iReport.
You can work around the issue by using callouts:
<property
name="ireport.callouts"
value="##Fri Nov 12 23:12:35 GMT-03:00 2010\\ncallouts.1.text=Hello World!\\ncallouts.1.bounds=294,80,181,73"/>