The tester has LED's and resisters between different pins on the outlet, different potentials between different pins light various LEDs. I suspect the ground (or I suppose any of the wires) on that circuit is questionable, and the connections in the tester is probably stabilizing it enough to pass whatever test the EVSE is doing. Check the tightness of all the wires on that outlet . If that doesn't help, you'd need to check any outlets that the circuit also goes to, as well as connections in the panel. It could even be a faulty appliance plugged into the circuit elsewhere, I suppose.