  Kagome metals possess peculiar optical spectra consisting of contributions
from free charge carriers in a Drude-type response, localized carriers seen as
a strongly temperature-dependent localization peak, and, in some cases, phonons
displaying strong anomalies. The rare-earth kagome metal series,
$R$Mn$_6$Sn$_6$, provides a marvelous playground to study the electronic
properties of kagome metals in the presence of variable magnetic order. Here,
we report temperature-dependent reflectivity studies on two members of the
$R$Mn$_6$Sn$_6$ family, GdMn$_6$Sn$_6$ (in-plane ferrimagnet) and
TbMn$_6$Sn$_6$ (out-of-plane ferrimagnet), in a broad energy range (50 - 18000
cm$^{-1}$, equivalent to 6.2 meV - 2.23 eV) down to 10 K. At high temperatures,
a phonon mode at approximately 160 cm$^{-1}$ is observed, which becomes
screened out in TbMn$_6$Sn$_6$ below $\sim$ 150 K as the localization peak
linearly passes through the mode. In GdMn$_6$Sn$_6$, the disappearance of the
phonon is accompanied by the onset of saturation of the peak position,
suggesting an unusual interplay between the two features.
