Sure sounds like a diode. Even if the diodes are fine, is there something causing a short between the lugs that the diode sits on. For instance, I had a switch that was missing the insulating paper between two blades, and it behaved exactly like a shorted diode even though the diode was fine.
Others seem to suggest an opto board issue. I’m not familiar with the game so that could very well be it, but I thought I would mention my experience in case it helps.