Magnets can fail and have internal shorts. You are going to have to disconnect one magnet at a time to determine which one is at fault.
In my trailer, if I ever have to do any brake wiring work, I will homerun every magnet to a common connection in a distribution box so that troubleshooting something like that is easier.
Use an ohmeter and measure between the brake terminal and ground of your trailer connector with it unplugged. Since magnets are typically a little over 3 ohms, you should measure slightly below 1 ohm for the 4 of them in parallel.