That does sound like incorrect behavior. If task B is completed, and task C was delayed because it uses the same resource as task B, then it should be moved earlier. Oh, unless there's a dependency between task B and task C. If that's the case, you'll need to enter task B's actual completion date before task C will adjust the way you'd expect.
If you're still seeing something that smells like a bug, feel free to send an email to our ninjas at
omniplan@omnigroup.com