  At $z=0$, higher-spin haloes with masses above
$\log(\text{M}_{\text{c}}/h^{-1}\text{M}_\odot)\simeq 11.5$ have a higher bias
than lower-spin haloes of the same mass. However, this trend is known to invert
below this characteristic crossover mass, $\text{M}_{\text{c}}$. In this paper,
we measure the redshift evolution and scale dependence of halo spin bias at the
low-mass end and demonstrate that the inversion of the signal is entirely
produced by the effect of splashback haloes. These low-mass haloes tend to live
in the vicinity of significantly more massive haloes, thus sharing their
large-scale bias properties. We further show that the location of the
redshift-dependent crossover mass scale $\text{M}_{\text{c}}(z)$ is completely
determined by the relative abundance of splashbacks in the low- and high-spin
subpopulations. Once splashback haloes are removed from the sample, the
intrinsic mass dependence of spin bias is recovered. Since splashbacks have
been shown to account for some of the assembly bias signal at the low-mass end,
our results unveil a specific link between two different secondary bias trends:
spin bias and assembly bias.
