Precision atomic spectroscopy provides a sensitive probe of physics beyond the Standard Model. A recently reported $9\sigma$ theory–experiment discrepancy in the ionization energy of metastable helium has motivated the hypothesis of a new boson mediating exotic electron–electron interactions. Using a general sign-consistency analysis of the induced energy shifts, we show that the sign requirement alone excludes vector–vector and pseudoscalar–pseudoscalar interactions as possible explanations of the anomaly. Incorporating existing constraints together with independent limits obtained in this work further excludes axial-vector scenarios. Within the single-boson framework considered in this work, only a narrowly constrained scalar-mediated interaction remains viable. The remaining parameter space could be probed in several different ways, for example, by modest improvements in the determination of the electron gyromagnetic ratio.