Inconsistent management of cacheKey and saveDocument

 // get the current doc
 DocumentModel doc = navigationContext.getCurrentDocument();

 // at this point doc.getCacheKey() = K1

 doc.setPropertyValue("dc:title", "changed title");
 DocumentModel modifiedDoc = documentManager.saveDocument(doc);

 // at this point doc.getCacheKey() = K2
 // at this point modifiedDoc.getCacheKey() = K2

 // here my problem is that I would expect doc.getCacheKey() == K1

 // because of that navigation won't work
 // since doc.getCacheKey()==modifiedDoc.getCacheKey()
 navigationContext.navigateToDocument(modifiedDoc);

 // in most of the cases this is not an issue since 
 // doc and modifiedDoc contains almost the same data
 // but for example the adapter cache is not correct