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In this study, we demonstrate the trapping of umbilical defects on a nematic liquid crystal cell using rectangular magnets. At equilibrium, we observe the formation of a stable vortex triplet. We propose a mechanism that explains the appearance of defects, focusing on the competition between two phenomena induced for the noise: one that generates vortex precursors and another that annihilates these precursors before they can fully develop into vortices. This mechanism is in accordance with both experimental and numerical observations.