Recent results suggest that flow-based algorithms may provide efficient
sampling of field distributions for lattice field theory applications, such as
studies of quantum chromodynamics and the Schwinger model. In this work, we
provide a numerical demonstration of robust flow-based sampling in the
Schwinger model at the critical value of the fermion mass. In contrast, at the
same parameters, conventional methods fail to sample all parts of configuration
space, leading to severely underestimated uncertainties.