MOSFET like relay for short-circuit
A glance at the IRL540N shows the reason for reverse conduction.



Figure 1. The MOSFET contains a built-in diode as part of the construction.

You can't add a diode because the voltage drop will be too high for your application.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 2. Modified circuit with built-in diodes shown.

You could add a second MOSFET in series but reversed. This would prevent either polarity passing unless the MOSFETS were turned on. The down-side is double the on resistance and the fact that the power circuit is not directly referenced to ground.