  We measure the coherent nonlinear response of excitons in a single-layer of
molybdenum disulphide embedded in hexagonal boron nitride, forming a
$h$-BN/MoS$_2$/$h$-BN heterostructure. Using four-wave mixing microscopy and
imaging, we correlate the exciton homogeneous and inhomogeneous broadenings. We
find that the exciton dynamics is governed by microscopic disorder on top of
the ideal crystal properties. Analyzing the exciton ultra-fast density dynamics
using amplitude and phase of the response, we investigate the relaxation
pathways of the resonantly driven exciton population. The surface protection
via encapsulation provides stable monolayer samples with low disorder, avoiding
surface contaminations and the resulting exciton broadening and modifications
of the dynamics. We identify areas localized to a few microns where the optical
response is totally dominated by homogeneous broadening. Across the sample of
tens of micrometers, weak inhomogeneous broadening and strain effects are
observed, attributed to the remaining interaction with the $h$-BN and
imperfections in the encapsulation process.
