  We postprocess a three-dimensional general relativistic, full transport
neutrino radiation magnetohydrodynamics simulation of the black hole--accretion
disk--wind system thought to be a potential outcome of the GW170817 merger to
investigate the presence of electron lepton number (ELN-XLN) crossings in the
neutrino angular distribution. Neutrinos are evolved with an explicit Monte
Carlo method and can interact with matter via emission, absorption, or
scattering. Within the postprocessing framework, we find ubiquitous occurrence
of ELN-XLN crossings at early times ($\sim$ 11ms) but this does not hold for
later times in the simulation. At postmerger times of $ \sim$ 60 ms and beyond,
ELN-XLN crossings are only present near the equator. We provide a detailed
analysis of the neutrino radiation field to investigate the origin and time
evolution of these crossings. Previous reports have suggested ubiquitous flavor
crossings persisting throughout the simulation lifetime, albeit for different
sets of conditions for the merger remnant, the treatment of hydrodynamics and
neutrino transport. Even though we do not perform a direct comparison with
other published works, we qualitatively assess the reasons for the difference
with our results. The geometric structure and evolution of the ELN-XLN
crossings found in our analysis, and by extension, fast flavor instabilities
have important implications for heavy element nucleosynthesis in neutron star
mergers.
