  We report a new, rare detection of HI 21-cm absorption associated with a
quasar (only six known at $1<z<2$) here towards J2339-5523 at $z_{em}$ =
1.3531, discovered through the MeerKAT Absorption Line Survey (MALS). The
absorption profile is broad ($\sim 400$ km/s), and the peak is redshifted by
$\sim 200$ km/s, from $z_{em}$. Interestingly, optical/FUV spectra of the
quasar from Magellan-MIKE/HST-COS spectrographs do not show any absorption
features associated with the 21-cm absorption. This is despite the coincident
presence of the optical quasar and the radio `core' inferred from a flat
spectrum component of flux density $\sim 65$ mJy at high frequencies ($>5$
GHz). The simplest explanation would be that no large HI column
(N(HI)$>10^{17}$ cm$^{-2}$) is present towards the radio `core' and the optical
AGN. Based on the joint optical and radio analysis of a heterogeneous sample of
16 quasars ($z_{median}$ = 0.7) and 15 radio galaxies ($z_{median}$ = 0.3) with
HI 21-cm absorption detection and matched in 1.4 GHz luminosity (L$_{\rm
1.4\,GHz}$), a consistent picture emerges where quasars are primarily tracing
the gas in the inner circumnuclear disk and cocoon created by the jet-ISM
interaction. These exhibit L$_{1.4\,\rm GHz}$ - $\Delta V_{\rm null}$
correlation, and frequent mismatch between the radio and optical spectral
lines. The radio galaxies show no such correlation and likely trace the gas
from the cocoon and the galaxy-wide ISM outside the photoionization cone. The
analysis presented here demonstrates the potential of radio spectroscopic
observations to reveal the origin of the absorbing gas associated with AGN that
may be missed in optical observations.
