GW231123: False Massive Graviton Signatures from Unmodeled Point-Mass Lensing

GW231123 is the strongest current candidate for a lensed gravitational wave event and a unique case for testing how point-mass lensing affects propagation-based tests of gravity. In the real GW231123 data, an unlensed IMRPhenomXPHM analysis yields an apparent nonzero graviton mass posterior. We show that this anomaly is naturally explained by unmodeled point-mass lensing: once lensing is included, the apparent graviton mass signal disappears. In GW231123-like injection-recovery tests, a lensed NRSur7dq4 signal with zero graviton mass, recovered with the same unlensed IMRPhenomXPHM template, produces a similarly pronounced spurious graviton mass posterior, whereas lensing-included analyses with IMRPhenomXPHM, IMRPhenomXO4a, and NRSur7dq4 remain mutually consistent with no evidence for nonzero graviton mass. The similarity between the injected and real data posteriors shows that unmodeled point-mass lensing can mimic modified gravitational wave propagation. These results identify a concrete failure mode in tests of gravity and strengthen the interpretation of GW231123 as a lensed candidate.