Oof, sorry, I think this particular question is over my head.
This sounds like this might be an issue of getting output printed in the right way.
I looked around and found this post: Mediation question using brms . There’s more documentation on that function here: https://strengejacke.github.io/sjstats/reference/mediation.html . Is that at all what you were looking for?
Other than that, you can replace your ‘brm’ call with make_stancode to generate the underlying Stan model brms uses (make_standata for the data) and verify brms is doing the thing you want.
Either of those things help? If not we can summon someone else to the thread who’ll know.