Help with modelling rstanarm model in brms #640

We are trying to fit a hierarchical bayesian survival model with a site-specific intercept and a site-specific coefficient for the covariate trt (i.e. treatment) using the brms and rstanarm packages. In rstanarm the model formulation for this is: formula = Surv(eventtime, status) ~ trt + (trt | site). We have tried several formulations in brms and are able to match the estimates however our credible intervals are completely different. This is whilst using the same priors, seed, chains, cores, iterations and adapt delta specifications. Can anyone tell us how to model the above rstanarm model in brms? Many thanks in advance