  On August 14, 2019, the LIGO and Virgo detectors observed GW190814, a
gravitational-wave signal originating from the merger of a $\simeq 23 M_\odot$
black hole with a $\simeq 2.6 M_\odot$ compact object. GW190814's
compact-binary source is atypical both in its highly asymmetric masses and in
its lower-mass component lying between the heaviest known neutron star and
lightest known black hole in a compact-object binary. If formed through
isolated binary evolution, the mass of the secondary is indicative of its mass
at birth. We examine the formation of such systems through isolated binary
evolution across a suite of assumptions encapsulating many physical
uncertainties in massive-star binary evolution. We update how mass loss is
implemented for the neutronization process during the collapse of the
proto-compact object to eliminate artificial gaps in the mass spectrum at the
transition between neutron stars and black holes. We find it challenging for
population modeling to match the empirical rate of GW190814-like systems whilst
simultaneously being consistent with the rates of other compact binary
populations inferred by gravitational-wave observations. Nonetheless, the
formation of GW190814-like systems at any measurable rate requires a supernova
engine model that acts on longer timescales such that the proto-compact object
can undergo substantial accretion immediately prior to explosion, hinting that
if GW190814 is the result of massive-star binary evolution, the mass gap
between neutron stars and black holes may be narrower or nonexistent.
