Unfortunately I’m not able to replicate the behaviour with the example you posted, see reprex below:

```
g_size <- c(7,6,30)
r_nb <- c(1.8, 2, 2.2) - .2
p_nb <- c(.4, .4, .4) + 0.1
obs <- unlist(mapply(FUN = rnbinom, g_size, r_nb, p_nb))
g_id = unlist(mapply(rep, 1:3, g_size))
df <- data.frame(y = obs, id = g_id)
barplot(with(df, table(id, y)))
library(brms)
#> Loading required package: Rcpp
#> Loading 'brms' package (version 2.15.0). Useful instructions
#> can be found by typing help('brms'). A more detailed introduction
#> to the package is available through vignette('brms_overview').
#>
#> Attaching package: 'brms'
#> The following object is masked from 'package:stats':
#>
#> ar
```

```
former = brm(bf(y ~ id, shape ~ 1, family = negbinomial),
df, chains = 1)
#> Compiling Stan program...
#> Start sampling
#>
#> SAMPLING FOR MODEL '627031144159c16cf193fb304e147739' NOW (CHAIN 1).
#> Chain 1:
#> Chain 1: Gradient evaluation took 3.6e-05 seconds
#> Chain 1: 1000 transitions using 10 leapfrog steps per transition would take 0.36 seconds.
#> Chain 1: Adjust your expectations accordingly!
#> Chain 1:
#> Chain 1:
#> Chain 1: Iteration: 1 / 2000 [ 0%] (Warmup)
#> Chain 1: Iteration: 200 / 2000 [ 10%] (Warmup)
#> Chain 1: Iteration: 400 / 2000 [ 20%] (Warmup)
#> Chain 1: Iteration: 600 / 2000 [ 30%] (Warmup)
#> Chain 1: Iteration: 800 / 2000 [ 40%] (Warmup)
#> Chain 1: Iteration: 1000 / 2000 [ 50%] (Warmup)
#> Chain 1: Iteration: 1001 / 2000 [ 50%] (Sampling)
#> Chain 1: Iteration: 1200 / 2000 [ 60%] (Sampling)
#> Chain 1: Iteration: 1400 / 2000 [ 70%] (Sampling)
#> Chain 1: Iteration: 1600 / 2000 [ 80%] (Sampling)
#> Chain 1: Iteration: 1800 / 2000 [ 90%] (Sampling)
#> Chain 1: Iteration: 2000 / 2000 [100%] (Sampling)
#> Chain 1:
#> Chain 1: Elapsed Time: 0.076959 seconds (Warm-up)
#> Chain 1: 0.090228 seconds (Sampling)
#> Chain 1: 0.167187 seconds (Total)
#> Chain 1:
latter = brm(bf(y ~ id, shape ~ 0, family = negbinomial),
df, chains = 1)
#> Compiling Stan program...
#> Start sampling
#>
#> SAMPLING FOR MODEL 'd123f231289692866b1610709a7e6d87' NOW (CHAIN 1).
#> Chain 1:
#> Chain 1: Gradient evaluation took 2.5e-05 seconds
#> Chain 1: 1000 transitions using 10 leapfrog steps per transition would take 0.25 seconds.
#> Chain 1: Adjust your expectations accordingly!
#> Chain 1:
#> Chain 1:
#> Chain 1: Iteration: 1 / 2000 [ 0%] (Warmup)
#> Chain 1: Iteration: 200 / 2000 [ 10%] (Warmup)
#> Chain 1: Iteration: 400 / 2000 [ 20%] (Warmup)
#> Chain 1: Iteration: 600 / 2000 [ 30%] (Warmup)
#> Chain 1: Iteration: 800 / 2000 [ 40%] (Warmup)
#> Chain 1: Iteration: 1000 / 2000 [ 50%] (Warmup)
#> Chain 1: Iteration: 1001 / 2000 [ 50%] (Sampling)
#> Chain 1: Iteration: 1200 / 2000 [ 60%] (Sampling)
#> Chain 1: Iteration: 1400 / 2000 [ 70%] (Sampling)
#> Chain 1: Iteration: 1600 / 2000 [ 80%] (Sampling)
#> Chain 1: Iteration: 1800 / 2000 [ 90%] (Sampling)
#> Chain 1: Iteration: 2000 / 2000 [100%] (Sampling)
#> Chain 1:
#> Chain 1: Elapsed Time: 0.057027 seconds (Warm-up)
#> Chain 1: 0.053828 seconds (Sampling)
#> Chain 1: 0.110855 seconds (Total)
#> Chain 1:
summary(former)
#> Family: negbinomial
#> Links: mu = log; shape = log
#> Formula: y ~ id
#> shape ~ 1
#> Data: df (Number of observations: 43)
#> Samples: 1 chains, each with iter = 2000; warmup = 1000; thin = 1;
#> total post-warmup samples = 1000
#>
#> Population-Level Effects:
#> Estimate Est.Error l-95% CI u-95% CI Rhat Bulk_ESS Tail_ESS
#> Intercept 0.61 0.59 -0.60 1.72 1.00 736 714
#> shape_Intercept 0.72 0.47 -0.09 1.77 1.00 640 418
#> id 0.09 0.22 -0.33 0.53 1.00 847 719
#>
#> Samples were drawn using sampling(NUTS). For each parameter, Bulk_ESS
#> and Tail_ESS are effective sample size measures, and Rhat is the potential
#> scale reduction factor on split chains (at convergence, Rhat = 1).
summary(latter)
#> Family: negbinomial
#> Links: mu = log; shape = log
#> Formula: y ~ id
#> shape ~ 0
#> Data: df (Number of observations: 43)
#> Samples: 1 chains, each with iter = 2000; warmup = 1000; thin = 1;
#> total post-warmup samples = 1000
#>
#> Population-Level Effects:
#> Estimate Est.Error l-95% CI u-95% CI Rhat Bulk_ESS Tail_ESS
#> Intercept 0.64 0.64 -0.55 1.92 1.00 1059 727
#> id 0.08 0.24 -0.38 0.54 1.00 1102 517
#>
#> Samples were drawn using sampling(NUTS). For each parameter, Bulk_ESS
#> and Tail_ESS are effective sample size measures, and Rhat is the potential
#> scale reduction factor on split chains (at convergence, Rhat = 1).
```

^{Created on 2021-06-18 by the reprex package (v2.0.0)}