It looks like a bug was entered recently:
https://redmine.pfsense.org/issues/15709

I'm having the same issue, except the bug states that this only happens when you've selected "Check to mail connection summary reports each log rotation interval" in the ZeekControl settings; however, I don't have that enabled and still receive the notification.

pfSense 2.7.2
Zeek 3.0.6_4