VJ is a parameter for the body diode in the MOSFET subcircuit. The manufacturer set it very high (likely a byproduct of curve fitting measurements). But if it’s set too high, the junction capacitance approaches infinity so ngspice limits it to avoid crashes/instability in the computation. It sets the limit at 1/FC where FC is another diode parameter and is set to 0.5 in this case (which is also the default).