  The true topological nature of the Kondo insulator SmB$_6$ remains to be
unveiled. Our previous tunneling study not only found evidence for the
existence of surface Dirac fermions, but it also uncovered that they inherently
interact with the spin excitons, collective excitations in the bulk. We have
extended such a spectroscopic investigation into crystals containing a Sm
deficiency. The bulk hybridization gap is found to be insensitive to the
deficiency up to 1% studied here, but the surface states in Sm-deficient
crystals exhibit quite different temperature evolutions from those in
stoichiometric ones. We attribute this to the topological surface states
remaining incoherent down to the lowest measurement temperature due to their
continued interaction with the spin excitons that remain uncondensed. This
result shows that the detailed topological nature of SmB$_6$ could vary
drastically in the presence of disorder in the lattice. This sensitiveness to
disorder is seemingly contradictory to the celebrated topological protection,
but it can be understood as being due to the intimate interplay between strong
correlations and topological effects.
