First and foremost check the connectors you are using between your design and the load's positive and negative terminals. Although on the board you may be measuring a 1V output, even a 1mOhm resistance across your wire connectors to the load would mean it is only seeing 0.925V at 75A which could be the limits of its specifications at that high a current. Very low resistance connectors such as copper bus bars would be recommended between a high current design and electronic load like this one.