This study investigates the sensitivity of radio wave propagation models to terrain sampling density in a 5G New Radio Vehicle-to-Everything downlink scenario at 3.6 GHz. Four widely used models are analysed: the empirical ITU-R P.1546-6, the deterministic Parabolic Equation Method, and the hybrid ITU-R P.1812-6 and ITU-R P.452-16. Real terrain profiles from Hungary are considered at multiple resolutions, allowing a systematic assessment of how accuracy degrades as the representation of terrain becomes coarser. The analysis reveals a consistent ranking across environments: the empirical model is the least affected by resolution changes, while deterministic and hybrid methods are significantly more sensitive. To interpret these differences, the study introduces a spectral complexity measure of terrain profiles and establishes its strong relationship with error growth through regression analysis. This provides a novel framework for explaining and quantifying the impact of terrain detail on model behaviour. The findings highlight both the methodological contribution of linking spectral complexity to propagation accuracy and the practical implications for optimising the trade-off between computational efficiency and prediction reliability in vehicular network planning.