No, it's definitely not a Unity bug, and Unity does not have any "built-in camera system," not like you apparently mean anyway. I know Unity very well and use it regularly. There are no camera bugs that could result in anything even remotely like "It is as if a camera capturing the scene had been inadvertently pointed to the lower right foreground and then slowly refocuses on the central image." A camera in Unity can't move on its own, and it can't "slowly refocus" on its own. That's 100% user code.
--Eric