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For decades, deuteron-based experiments established a canonical SU(2) flavor asymmetry in the proton’s light sea, with more $\bar{d}$ than $\bar{u}$ antiquarks, interpreted as a non-perturbative signature. However, these results rely on nuclear assumptions and isospin symmetry. Here, we analyze pure proton collision data from the Fermilab Tevatron and CERN LHC using measurements of Drell–Yan forward–backward asymmetries, free of nuclear corrections. We find that the proton’s $\bar{d}/\bar{u}$ ratio is consistent with unity at $x\sim 0.1$, contradicting the long-standing asymmetry mentioned above. This suggests a symmetric, perturbative origin for light sea antiquarks, implying the deuteron anomaly arises from high-energy nuclear effects rather than intrinsic proton asymmetry. Implications for proton structure and global PDF analyses are also discussed.