  The scale-dependent bias of galaxy density contrasts is an important signal
to be extracted in constraining local primordial non-Gaussianity ($f_{\rm
NL}^{\text{local}}$) from observations of large-scale structure. Constraints so
obtained rely on the assumption that horizon-scale features in the galaxy power
spectrum are exclusively due to primordial physical mechanisms. Yet,
post-inflationary effects can induce modulations to the galaxy number density
that appear as horizon-scale, scale-dependent bias. We investigate the effect
of two such sources of scale-dependent bias - the free-streaming of light
relics and fluctuations in the background of ionising radiation - on precision
measurements of local primordial non-Gaussianity $f_{\rm NL}^{\text{local}}$
from galaxy power spectrum measurements. Using the SPHEREx survey as a test
case survey reaching $\sigma(f_{\rm NL}^{\rm local}) \lesssim 1$, we show that
ignoring the scale-dependent bias induced by free-streaming particles can
negatively bias the inferred value of $f_{\rm NL}^{\rm local}$ by $\sim
0.1-0.3\sigma$. Ignoring the effect of ionising radiation fluctuations can
negatively bias the inferred value of $f_{\rm NL}^{\rm local}$ by $ \sim
1\sigma$. The range of biases depends on the source populations and the ranges
of scales used in the analysis, as well as the value of the neutrino mass and
the modelling of the impact of ionising radiation. If these sources of
scale-dependent bias are included in the analysis, forecasts for $f_{\rm
NL}^{\rm local}$ are unbiased but degraded.
