  The production rate of $B^{0}_{s}$ mesons relative to $B^{0}$ mesons is
measured by the LHCb experiment in $pp$ collisions at a center-of-mass energy
$\sqrt{s} = 13$ TeV over the forward rapidity interval $2<y<4.5$ as a function
of the charged particle multiplicity measured in the event. Evidence at the
3.4$\sigma$ level is found for an increase of the ratio of $B^{0}_{s}$ to
$B^{0}$ cross-sections with multiplicity at transverse momenta below 6 GeV/$c$,
with no significant multiplicity dependence at higher transverse momentum.
Comparison with data from $e^{+}e^{-}$ collisions implies that the density of
the hadronic medium may affect the production rates of $B$ mesons. This is
qualitatively consistent with the emergence of quark coalescence as an
additional hadronization mechanism in high-multiplicity collisions.
