I received a docx document the other day, opened it in Writer and started reviewing it. I added a lot of comments, which BTW looked very good. I saved the document and closed Writer. I then reopened the document, no comments. Everything gone. A lot of cursing ensued and then I filed this report.
I have had the same problem (since OOo 3.3). For Marc: Recently I tried to open the file in Word 2010, and it appears that the comments were visible, even if not visible in LibreOffice. Might be worth a try.
*** This bug has been marked as a duplicate of bug 33463 ***