Thanks for your inquiry. I have managed to reproduce the same issue at my side. I have logged this issue as WORDSNET-7715 in our issue tracking system. I have linked this forum thread to the same issue and you will be notified via this forum thread once this issue is resolved. We apologize for your inconvenience.
Please use the following code snippet as a workaround. First get the value of Document.TrackRevisions property after loading the document and set it back after calling Document.Unprotect method as shown below.