  Residual foreground contamination by thermal Sunyaev-Zeldovich (tSZ) effect
from galaxy clusters in cosmic microwave background (CMB) maps propagates into
the reconstructed CMB lensing field, and thus biases the intrinsic
cross-correlation between CMB lensing and large-scale structure (LSS). Through
stacking analysis, we show that residual tSZ contamination causes an increment
of lensing convergence in the central part of the clusters and a decrement of
lensing convergence in the cluster outskirts. We quantify the impact of
residual tSZ contamination on cross-correlations between the Planck 2018 CMB
lensing convergence maps and the SDSS-IV galaxy density data through
cross-power spectrum computation. In contrast with the Planck 2018
tSZ-deprojected SMICA lensing map, our analysis using the tSZ-contaminated
SMICA lensing map measures a $\sim2.5\%$ negative bias at multipoles
$\ell\lesssim 500$ and transits to a $\sim9\%$ positive bias at
$\ell\gtrsim1500$, which validates earlier theoretical predictions of the
overall shape of such tSZ-induced spurious cross-correlation. The tSZ-induced
lensing convergence field in Planck CMB data is detected with more than
$1\sigma$ significance at $\ell\lesssim 500$ and more than $14\sigma$
significance at $\ell\gtrsim1500$, yielding an overall $14.8\sigma$ detection.
We also show that masking galaxy clusters in CMB data is not sufficient to
eliminate the spurious lensing signal, still detecting a non-negligible bias
with $5.5\sigma$ significance on cross-correlations with galaxy density fields.
Our results emphasize how essential it is to deproject the tSZ effect from CMB
maps at the component separation stage and adopt tSZ-free CMB lensing maps for
cross-correlations with LSS data.
