BYD (1211.HK): Long AI, Short Autos? Or A Repeat of Lagged Mean-Reversion? (Mid-Sep Update)

BYD share price seemingly disconnects with its fundamentals, but to a lesser magnitude than in 2Q26: BYD share price in Aug and MTD-Sep declined by -7%/-8%, yet Citi estimates BYD's weighted-fundamental index improved to 0.02 in Aug and further surged to 0.09 by MTD-Sep versus May's 0.08 level. This mismatch appears a repeat of the 2Q26 cycle but milder and in the opposite direction. A mild mean-reversion between BYD share price and Citi's AI fund flow index (Weichai/Generac/Bloom Energy) has just started since 10-Sep, with BYD +1% versus the index -2%, similar to July. Valuation is declining faster than earnings revisions: since 2Q results FY26/27E consensus earnings have risen, so the share price decline implies weaker confidence in valuation multiples following the Aug sector retail sales miss, Sep orders miss, news of a potential export tax rebate cut and possible higher EU tariff on PHEV. Citi remains constructive: any export tax rebate cut would likely be gradual at around -4ppt