Hi, sorry for not getting to you earlier. Did you manage to resolve the issue in the meantime?
For me, the question as written is a bit too vague to answer, especially this:
Why do you think it is not correct? What do the plots look like? Generally, point hypotheses are a weird bunch, see e.g. Hypothesis testing, model selection, model comparison - some thoughts. In the light of the Schad et al. Bayes factor workflow paper I would also not be surprised if the default method for computing BF in brms which AFAIK is Savage-Dickey just failed for this specific hypothesis, as it AFAIK is intended only for hypotheses of the type something = 0.
Best of luck with your model