Simulink "ideal switch" block changes output unexpectedly
The reason for what you are seeing is the background engine is attempting to resolve the sudden change in current (abrupt breaker opening) against is mathematical representation of a DC machine.

If you look under the mask of the DC machine you will see the A+ and A- terminals are linked via an ideal-R and an ideal-L  plus a voltage source. It then measures the "stator current" to then influence the mathematical model w.r.t. the backemf:  Torque = Kt*i => inertia => integrator etc...

The only way to mathematically resolve the sudden stop in current is with the apparent high current to "model" a massive deceleration.

This resistor provides a freewheel path for the model's current