  A key result of isotropic loop quantum cosmology is the existence of a
quantum bounce which occurs when the energy density of the matter field
approaches a universal maximum close to the Planck density. Though the bounce
has been exhibited in various matter models, due to severe computational
challenges some important questions have so far remained unaddressed. These
include the demonstration of the bounce for widely spread states, its detailed
properties for the states when matter field probes regions close to the Planck
volume and the reliability of the continuum effective spacetime description in
general. In this manuscript we rigorously answer these questions using the
Chimera numerical scheme for the isotropic spatially flat model sourced with a
massless scalar field. We show that as expected from an exactly solvable model,
the quantum bounce is a generic feature of states even with a very wide spread,
and for those which bounce much closer to the Planck volume. We perform a
detailed analysis of the departures from the effective description and find
some expected, and some surprising results. At a coarse level of description,
the effective dynamics can be regarded as a good approximation to the
underlying quantum dynamics unless the states correspond to small scalar field
momenta, in which case they bounce closer to the Planck volume, or are very
widely spread. Quantifying the amount of discrepancy between the quantum and
the effective dynamics, we find that the departure between them depends in a
subtle and non-monotonic way on the field momentum and different fluctuations.
Interestingly, the departures are generically found to be such that the
effective dynamics overestimates the spacetime curvature, and underestimates
the volume at the bounce.
