The brightest gamma-ray burst (GRB) ever recorded, GRB 221009A, provides a crucial opportunity to understand the nature of GRBs. In this work, we propose an inevitable yet previously overlooked phenomenon occurring during the early afterglow phase of this GRB. We investigate the GeV-range light curve resulting from external inverse Compton (EIC) emission seeded by the prompt emission, taking into account the effects of pair loading and preacceleration of the ambient medium induced by the prompt photons. Through a broad parameter survey, we derive general constraints on the ambient density profile. We also evaluate the consistency of previously proposed afterglow models with the observed GeV light curve. Our results show that some of these models significantly overshoot the observations, indicating a possible inconsistency. This study provides insights into the jet structure and the surrounding environment of GRB 221009A. In the seminar, I will present our recent results, the ongoing extension, and my perspective.