Finally had some time to call the power company (Eversource). They were here within an hour of my call. Pulled the meter and found a broken neutral at the connection by the road (I have buried service). This neutral was shared with the neighbor so it seems like we collectively had a floating neutral between the two houses. It might be a useful data point for the floating neutral lab to take a look at the data for future enhancements. In a few days when the lab updates based on the repair I’ll share the new charts.