A Hexagonal Counterexample to Log-Convexity of Fisher Information Along the Heat Flow

We construct a smooth, strictly positive, Gaussian-decaying density on $\R^2$ for which Fisher information along the heat flow is not log-convex. This disproves the Cheng--Geng log-convexity conjecture in dimension two and, by tensorization, in every dimension $d\ge2$. Consequently, the multidimensional forms of the Gaussian completely monotone conjecture, McKean's conjecture, and Toscani's entropy power conjecture also fail. This complements the recent one-dimensional counterexample of Gu and Sellke. The counterexample is a small hexagonal perturbation on the triangular torus, transferred to $\R^2$ by a Gaussian envelope, and supported by explicit two-dimensional numerics. Finally, we initiate the study of the sharp constants $θ_d^*$ by proving $θ_1^*=1$, establishing monotonicity in the dimension, and recording an intrinsic simplex resonance family in every fixed dimension. The explicit two-dimensional counterexample was found by GPT-5.5 Pro.