The issue was caused by a barrier terminal strip used to daisy chain 8 connections. Each jumper feed between positions had 2 spade connections, so from the first to the last there were 8 screw connections and 16 crimps in series on the feed side, plus the crimp and screw on the load side. At less than one amps I measured a ~200 millivolt drop in this assembly.

I will pick up a bus bar to simplify this. While there are other drops, this stands out as a major offender.