  We report the emergence of a new HI 21-cm absorption at z_abs = 1.172635 in
the damped Lyman-alpha absorber (DLA) towards the gamma-ray blazar PKS 2355-106
(z_em~1.639) using science verification observations (June 2020) from the
MeerKAT Absorption Line Survey (MALS). Since 2006, this DLA is known to show a
narrow HI 21-cm absorption at z_abs = 1.173019 coinciding with a distinct metal
absorption line component. We do not detect significant HI 21-cm optical depth
variations from this known HI component. A high resolution optical spectrum
(August 2010) shows a distinct Mg I absorption at the redshift of the new HI
21-cm absorber. However, this component is not evident in the profiles of
singly ionized species. We measure the metallicity ([Zn/H] = -(0.77\pm0.11) and
[Si/H]= -(0.96\pm0.11)) and depletion ([Fe/Zn] = -(0.63\pm0.16)) for the full
system. Using the apparent column density profiles of Si II, Fe II and Mg I we
show that the depletion and the N(Mg I)/N(Si II) column density ratio
systematically vary across the velocity range. The region with high depletion
tends to have slightly larger N(Mg I)/N(Si II) ratio. The two HI 21-cm
absorbers belong to this velocity range. The emergence of z_abs = 1.172635 can
be understood if there is a large optical depth gradient over a length scale of
~0.35 pc. However, the gas producing the z_abs = 1.173019 component must be
nearly uniform over the same scale. Systematic uncertainties introduced by the
absorption line variability has to be accounted for in experiments measuring
the variations of fundamental constants and cosmic acceleration even when the
radio emission is apparently compact as in PKS 2355-106.
