Ok, here's an update:
I just tried re-flashing the firmware on the fujinet device thinking maybe I got a bad install from the factory. After completing, I get the same behavior as before where it's like I'm holding down return, so I doubt it's a firmware problem. Also, I just noticed that on the unit I have, there is a cutout for the On/Off switch, but the switch isn't installed. Was this a design change, or did I get a defective unit and that's causing the problem? I've sent a message to Arcade Shopper where I purchased it from, but haven't got a response yet.