The probability of an ALP decaying invisible is $$\mathrm{BR}(B^+\to K^++\mathrm{inv}) = \mathrm{BR}(B^+\to K^+ a) \times P_\mathrm{out},$$ with $$P_\mathrm{out}= \exp\left(-\frac{L_\mathrm{det}}{c\tau_a \, \beta_a \gamma_a}\right)\,.$$ The difference in detector size and boost between BaBar and Belle II allows us to disentangle \(\mathrm{BR}(B^+\to K^+ a)\) and \(P_\mathrm{out}\).
For \(m_a = 2\,\mathrm{GeV}\), preference for longer-lived ALP and \(\mathrm{BR}(B^+\to K^+ a) \sim 5\times 10^{-6}\).