I noticed this odd behavior with the traffic graphs on the Lobby:Dashboard page. I was able to observe it in multiple OPNSense firewalls of differing devices. The version I am running (everywhere) is 25.1.10-amd64.
When the tab/window isn't in view, the stats drop down to 0. Then when you return to the window it recovers back to it's normal rate.
This is most likely your browser putting that tab on hold while it is in the background.