The UMC doesn't know 120v from 240v - my tests for the 20A maximum are based upon me moving the neutral to a hot pin and presenting 120V on a NEMA 14-50. The failure you got was odd, though -- when I reverse polarity, I get a red code for ground fault. I suspect Tesla is measuring ground vs. that one specific hot to make sure they're not at same potential (roughly). So no idea what triggered the 8 blinks...