Although Mn²⁺ additives alleviate the dissolution issue of Mn-based cathodes in aqueous zinc–ion batteries (ZIBs), problems including complex side reactions and abnormal capacity fluctuation pose new challenges for their large-scale applications. In virtue of manganese oxide based on preeminent cation-pillar engineering, we elaborate the origin of capacity fluctuation, which is found to be correlated with the unique Mn²⁺ behavior. For the first time, we figure out new metrics such as effective cycling percentage (η) and maximum Mn²⁺ contribution ratio (ε) to reappraise the electrochemical performance of the current Mn-based ZIBs via a new capacity evaluation protocol, i.e., Mn-based competitive capacity evolution (Mn-CCE). The universality of the protocol and its metrics were further verified via quantitative analyses of the reported Mn-based ZIBs. More significantly, the failure of Mn-based electrodes is demonstrated to be rescuable via facile acid treatment, which is expected to quintuple the lifespan of batteries. The findings can provide new insights to understand the electrochemical behaviors, serve as the assessment criteria, and further guide the development of Zn/Mn-related devices for practical applications.