<p>Agricultural product crowdfunding increasingly relies on joint brand building by platforms and farmers. However, many platforms reinvest remaining escrowed funds and may exclusively capture the returns. This arrangement can generate fairness loss for farmers, suppress branding effort, and trigger exit, which threatens cooperation stability. Against this background, the research question concerns how the secondary investment return structure shapes farmers fairness loss and exit incentives, and how alternative contracts can mitigate cooperation breakdown. Methodologically, the Stackelberg differential game is developed with the platform as the leader and fairness concerned farmers as followers. The model tracks the dynamic evolution of brand reputation and derives effort, profit, and threshold exit outcomes under farsighted and shortsighted strategies. Two extensions are considered, including cost sharing of farmers branding costs and revenue sharing of secondary investment returns. Numerical analysis examines the roles of service fees; remaining fund transfer periods, information transparency, and secondary investment return rates. Key findings show that brand reputation increases demand and performance. Profit incentives make the platform prefer a farsighted strategy, but power imbalance breaks the alignment between brand improvement and farmer profit. Under high transparency and high platform investment capability, accumulated fairness loss can push cooperation into a prisoners’ dilemma. Cost sharing raises platform profit and brand indicators but does not materially improve farmer profit. Revenue sharing transfers incremental gains to farmers and substantially reduces exit risk. The contribution lies in clarifying the dynamic coupling among branding, allocation rules, and cooperation stability, characterizing a platform capability curse, and motivating staged allocation and governance design.</p>