We study symmetric-cube hypergeometric coefficients at the mixed CM point (1/6, 1/3;1) on X₀ (3). The full split-prime supercongruence A (mp) ≡ A (m) (mod p⁴) is reduced to a single scalar congruence Aₚ^mix ≡ 18 (mod p⁴). This scalar is evaluated unconditionally modulo p (Wolstenholme–Fermat–unit root), modulo p² (Eisenstein–Lerch endpoint theorem), and modulo p³ (Clausen convolution decomposition with Pfaff–Saalschütz evaluation and sixth-class harmonic matching). The mod p³ cancellation is exact: the endpoint term uₚ and the middle correction Wₚ carry the same Θₚ digit with opposite signs. The remaining open level is mod p⁴.
Alex Shvets (Wed,) studied this question.