The claim that magnetic monopoles violate U(1) gauge symmetry is incorrect; rather, magnetic monopoles are routinely formulated within U(1) gauge theories and grand unified gauge frameworks, as demonstrated by the Dirac quantization condition.
The retrieved literature (such as papers 5, 8, and 10) explicitly treats magnetic monopoles as consistent with or derived from U(1) gauge frameworks (e.g., standard quantum electrodynamics, U(1)_em gauge symmetry, or Dirac quantization conditions). Therefore, the claim that their existence violates U(1) gauge symmetry is refuted.