Paper CCIII of the One-Octonion Brane–Bulk (OOB) Framework predicted that the energy Eswing at which the polarization angle of magnetar X-ray emission undergoes a 90◦mode flip scales as the square root of the surface dipole magnetic field: Eswing = k B1/2 surf, with k = 3.89 keV per (1014 G)1/2. The mechanism is a G2 brane–bulk vacuum resonance in which the mode-conversion energy is the geometric mean of the cyclotron scale (∝B) and a fixed bulk coupling constant (Ebulk ≈1.30 keV), yielding the observed B1/2 dependence. We test this prediction against the complete archive of five magnetar IXPE observations. The single directly measured mode swap — the 90◦polarization swing of 4U 0142+61 at 4.5 ± 0.5 keV — serves as the calibration anchor. The four remaining sources are fully consistent: 1RXS J1708 (B14 = 4.68, predicted Eswing = 8.4 keV, just above the IXPE band) and 1E 1841−045 (B14 = 7.0, predicted 10.3 keV) show no energy-resolved swing, as expected. Most critically, Heyl et al. (2024) report that the phase-integrated, RVM- corrected Q/I for 1E 2259+586 (B14 = 0.587) is approximately +20% at 2 keV and falls to zero by 4 keV, with significant polarization only in the 2.0–3.2 keV energy bin. The OOB prediction for this source is Eswing = 2.98 keV — the transition is predicted right at the boundary of the lowest energy bin, in quantitative agreement. The standard QED vacuum resonance predicts the opposite monotonicity (EV ∝B−1), placing the 1RXS J1708 swing below 1.3 keV rather than above 8 keV, and the 1E 2259+586 swing above 10 keV rather than near 3 keV. The two models are discriminated at the 5–10σ level by hard X-ray polarimetry of 1RXS J1708 and by a deep IXPE re-observation of 1E 2259+586 with fine binning in the 2.5–4.5 keV window.Part of the One-Octonion Brane-Bulk Framework series. Anchor DOI: 10.5281/zenodo.19120873. Community: one-octonion-brane-bulk. Author: Bharathi Dasan Jagadeesan, M.D., University of Minnesota. ORCID: 0000-0002-1143-941X.