  Determining the type of excitations that can exist in a thermal medium is key
to understanding how hadronic matter behaves at extreme temperatures. In this
work we study this question for pseudo-scalar mesons comprised of light-strange
and strange-strange quarks, analysing how their low-energy spectral properties
are modified as one passes through the high-temperature chiral crossover region
between $T=145.6 \, \text{MeV}$ and $172.3 \, \text{MeV}$. We utilise the
non-perturbative constraints satisfied by correlation functions at finite
temperature in order to extract the low-energy meson spectral function
contributions from spatial correlator lattice data in $N_{f}=2+1$ flavour QCD.
The robustness of these contributions are tested by comparing their predictions
with data for the corresponding temporal correlator at different momentum
values. We find that around the pseudo-critical temperature $T_{\text{pc}}$ the
data in both the light-strange and strange-strange channels is consistent with
the presence of a distinct stable particle-like ground state component, a
so-called thermoparticle excitation. As the temperature increases this
excitation undergoes collisional broadening, and this is qualitatively the same
in both channels. These findings suggest that pseudo-scalar mesons in QCD have
a bound-state-like structure at low energies within the chiral crossover region
which is still strongly influenced by the vacuum states of the theory.
