Negative voltage across ground points under load
As discussed in the comments, you have about a 0.5 &Omega; resistance in your ground wiring. You are losing about 0.6 volts when under load. Ohm's Law (V=I*R ==> I=V/R) says I=~0.6/~0.5 = about 1.2 amps flowing through that ground. It all adds up.

Fix the resistance in the ground wiring and you'll fix your voltage drop.