Packet Loss Bug?
I don't see this noted any place else, but before I submit it as a bug I thought I should check to see if it is a know problem.
I use fixed wiireless for my internet connection which is susceptible to weather problems. During a recent ice storm, the antenna coated with ice and the connection started loosing packets to the point of being unusable. At first the gateway status reported the loss as something between 1 and 100% but after some time, the packet loss stated exceeding 100% and reached 248%. Once the ice melted, and the connection stabilized the packet loss number remained at 188% over a number of days until I restarted pfSense.
Not seen it with packet loss but I know a similar thing has been reported for ping time. As your screen shot shows the ping time can start to accumulate to a ludicrous figure. I imagine they are related.
You can restart the monitoring daemon (Status > Services) and it will wipe out the old loss data.
We have seen that happen before but have not been able to replicate the circumstances that result in the problem.