Running 12.0 Beta, but this was present in 11.2 and 11.3 as well. To reproduce, navigate to dashboard, click out of browser to make the interface no longer active, wait a few seconds, then click back in. The next update of the interface throughput stats show crazy values (shown below) before being updated again to sane values. I've got this running on a lagg interface, but I'm not sure if that matters. Obviously, it has something to do with interface updates being paused when window is no longer active.