  We use inelastic neutron scattering to study spin fluctuations in
Sr$_{1.8}$La$_{0.2}$RuO$_4$, where Lanthanum doping triggers a Lifshitz
transition by pushing the van Hove singularity in the $\gamma$ band to the
Fermi energy. Strong spin fluctuations emerge at an incommensurate wave vector
$\mathbf{Q}_{ic} = (0.3,0.3)$, corresponding to the nesting vector between
$\alpha$ and $\beta$ Fermi sheets. The incommensurate antiferromagnetic
fluctuations shift toward $(0.25,0.25)$ with increasing energy up to
${\sim}110$ meV. By contrast, scatterings near the ferromagnetic wave vectors
$\mathbf{Q} = (1,0)$ and $(1,1)$ remain featureless at all energies. This
contradicts the weak-coupling perspective that suggests a sharp enhancement of
ferromagnetic susceptibility due to the divergence of density of states in the
associated $\gamma$ band. Our findings imply that ferromagnetic fluctuations in
Sr$_2$RuO$_4$ and related materials do not fit into the weak-coupling paradigm,
but instead are quasi-local fluctuations induced by Hund's coupling. This
imposes significant constraints for the pairing mechanism involving spin
fluctuations.
