We investigate how the transition density (${\rho }_{\mathrm{tr}}$) affects hybrid constructions of the neutron star (NS) equation of state (EOS) in which a nucleonic description at low densities is matched to a model-agnostic high-density extension based on a speed-of-sound (CS) parametrization. Using four representative nucleonic models—Taylor expansion, $\frac{n}{3}$ expansion, Skyrme, and relativistic mean field—built from identical nuclear matter parameters, we isolate the impact of the low-density EOS and the transition density on NS observables. We find that, within the present smooth-matching prescription, NS properties such as radii and tidal deformabilities retain significant sensitivity to the choice of low-density EOS for commonly adopted transition densities around ${\rho }_{\mathrm{tr}}\approx 2{\rho }_0$, even when the same high-density parametrization is employed. This residual dependence arises from differences in the matching conditions at ${\rho }_{\mathrm{tr}}$, which propagate into the high-density extension, so different low-density inputs lead to different effective high-density EOS. These findings are robust across two qualitatively distinct CS parametrizations differing in peak height, location, and width. Quantitatively, the model spread in radius and tidal deformability at $1.4M_{\odot }$ exceeds the current observational uncertainty (${\sigma }_{R_{1.4}}=0.8\mathrm{km}$ and ${\sigma }_{{\mathrm{\Lambda }}_{1.4}}=200$) by factors of $\sim 1.8$ and $\sim 1.4$ at ${\rho }_{\mathrm{tr}}\approx 2{\rho }_0$, whereas these factors reduce to $\sim 1.05$ and $\sim 0.4$ at ${\rho }_{\mathrm{tr}}={\rho }_0$. Lowering the transition density, therefore, systematically diminishes the spread among models and leads to more consistent predictions. Our results demonstrate that the widely used choice ${\rho }_{\mathrm{tr}}\approx 2{\rho }_0$ does not guarantee practical model independence within the present hybrid EOS framework and should be treated as an explicit source of systematic uncertainty when inferring dense matter properties from NS observations.

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• Gravitation
• Gravitational waves
• Nuclear astrophysics
• Nuclear matter
• Nuclear matter in neutron stars