  The absence of odd-order Shapiro steps is one of the predicted signatures for
topological superconductors. Experimentally, the missing first-order Shapiro
step has been reported in several superconducting systems presumably to be
topologically non-trivial, as well as in the topologically trivial regime of
superconductor-semiconductor Josephson junctions. In this work, we revisit the
missing first Shapiro step signature in the topologically trivial regime of
Al-InSb nanosheet Josephson junctions under microwave irradiation. The missing
first Shapiro step is found to be accompanied by a sharp voltage jump during
the superconducting switching and reappears when the jump is softened by
increasing temperature or magnetic field. The missing first Shapiro step also
reappears with an increased microwave frequency. The sharp switching jump,
existing without microwave irradiation, deviates from the relation given by the
standard resistively shunted junction (RSJ) model. Missing Shapiro step
signatures are qualitatively captured by introducing the sharp voltage jump
into the RSJ model. This work reveals a common, yet overlooked, phenomenon that
leads to the missing first Shapiro step, providing a new perspective on
fractional Josephson experiments.
