Update when log message changes
That's the expected behavior: SVN does not keep a record on when log messages are edited. So CM does not get a notification about that. Since it has already fetched that message, it does not know that it has changed.
You can select that message, hit the Del key to delete it and then click the "Check now" button - CM will re-fetch that (now missing) log entry with the new message.
CommitMonitor
If someone changes a log message it is not updated in CommitMonitor.