  Next-generation cosmological surveys will observe larger cosmic volumes than
ever before, enabling us to access information on the primordial Universe, as
well as on relativistic effects. In a companion paper, we applied a Fisher
analysis to forecast the expected precision on $f_{\rm NL}$ and the
detectability of the lensing magnification and Doppler contributions to the
power spectrum. Here we assess the bias on the best-fit values of $f_{\rm NL}$
and other parameters, from neglecting these light-cone effects. We consider
forthcoming 21cm intensity mapping surveys (SKAO) and optical galaxy surveys
(DESI and Euclid), both individually and combined together. We conclude that
lensing magnification at higher redshifts must be included in the modelling of
spectroscopic surveys. If lensing is neglected in the analysis, this produces a
bias of more than 1$\sigma$ - not only on $f_{\rm NL}$, but also on the
standard cosmological parameters.
