  We present NOEMA CO (2-1) line and ALMA 870 $\mu$m continuum observations of
a main-sequence galaxy at $z=1.38$. The galaxy was initially selected as a
"gas-giant", based on the gas mass derived from sub-mm continuum (log$(M_{\rm
gas}/M_{\odot})=11.20\pm0.20$), however the gas mass derived from CO (2-1)
luminosity brings down the gas mass to a value consistent with typical
star-forming galaxies at that redshift (log$(M_{\rm
gas}/M_{\odot})=10.84\pm0.03$). Despite that the dust-to-stellar mass ratio
remains elevated above the scaling relations by a factor of 5. We explore the
potential physical picture and consider an underestimated stellar mass and
optically thick dust as possible causes. Based on the updated gas-to-stellar
mass ratio we rule out the former, and while the latter can contribute to the
dust mass overestimate it is still not sufficient to explain the observed
physical picture. Instead, possible explanations include enhanced HI
reservoirs, CO-dark H$_2$ gas, an unusually high metallicity, or the presence
of an optically dark, dusty contaminant. Using the ALMA data at 870 $\mu$m
coupled with $HST$/ACS imaging, we find extended morphology in dust continuum
and clumpy star-formation in rest-frame UV in this galaxy, and a tentative
$\sim 10$ kpc dusty arm is found bridging the galaxy center and a clump in
F814W image. The galaxy shows levels of dust obscuration similar to the
so-called $HST$-dark galaxies at higher redshifts, and would fall into the
optically faint/dark $JWST$ color-color selection at $z>2$. It is therefore
possible that our object could serve as low-$z$ analog of the $HST$-dark
populations. This galaxy serves as a caveat to the gas masses based on the
continuum alone, with a larger sample required to unveil the full picture.
