init list is duplicating inits rather than using all of them

Summary:

When supplying a list of 4 lists for initialization and specifying chains = 4, the 1st element is not used and the 4th element is duplicated.

Reproducible Steps:

library(rstan)

binormal_model <- stan_model('binormal.stan')
inits <- list(list(theta = c(8, 8)), list(theta = c(8, -8)),
              list(theta = c(-8, 8)), list(theta = c(-8, -8)))
its <- 30
fit <- sampling(binormal_model, init = inits, chains = 4, iter = its,
                control = list(stepsize = 1e-3), seed = 1234)
draws <- extract(fit, permuted = FALSE, inc_warmup = TRUE, pars = c("theta"))
df1 <- data.frame(theta1 = draws[ , 1, 1], theta2 = draws[ , 1, 2])
df2 <- data.frame(theta1 = draws[ , 2, 1], theta2 = draws[ , 2, 2])
df3 <- data.frame(theta1 = draws[ , 3, 1], theta2 = draws[ , 3, 2])
df4 <- data.frame(theta1 = draws[ , 4, 1], theta2 = draws[ , 4, 2])

traceplot <- traceplot(fit, inc_warmup = TRUE)

library(ggplot2)
traceplot2d <-
  ggplot(df1, aes(x = theta1, y = theta2)) +
  geom_path(data = df1, color = "red") +
  geom_path(data = df2, color = "yellow") +
  geom_path(data = df3, color = "blue") +
  geom_path(data = df4, color = "green")

Current Output:

The first init, (8, 8), is not used, and the fourth init, (-8, -8), is used twice. Here's the traceplot and the 2D traceplot:

Expected Output:

The four inits given to be used.

RStan Version:

> sessionInfo()
R version 3.5.0 (2018-04-23)
Platform: x86_64-apple-darwin15.6.0 (64-bit)
Running under: macOS  10.15.2

Matrix products: default
BLAS: /Library/Frameworks/R.framework/Versions/3.5/Resources/lib/libRblas.0.dylib
LAPACK: /Library/Frameworks/R.framework/Versions/3.5/Resources/lib/libRlapack.dylib

locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] rstan_2.19.2          ggplot2_3.1.0         StanHeaders_2.18.1-10

loaded via a namespace (and not attached):
 [1] Rcpp_0.12.18       pillar_1.2.3       compiler_3.5.0     plyr_1.8.4        
 [5] bindr_0.1.1        prettyunits_1.0.2  base64enc_0.1-3    tools_3.5.0       
 [9] digest_0.6.15      pkgbuild_1.0.2     tibble_1.4.2       gtable_0.2.0      
[13] pkgconfig_2.0.2    rlang_0.4.0        cli_1.0.0          parallel_3.5.0    
[17] loo_2.0.0          bindrcpp_0.2.2     gridExtra_2.3      withr_2.1.2       
[21] dplyr_0.7.6        stats4_3.5.0       grid_3.5.0         tidyselect_0.2.4  
[25] glue_1.2.0         inline_0.3.15      R6_2.2.2           processx_3.2.0    
[29] callr_3.0.0        purrr_0.2.5        magrittr_1.5       scales_0.5.0      
[33] ps_1.2.0           codetools_0.2-15   matrixStats_0.54.0 assertthat_0.2.0  
[37] colorspace_1.3-2   labeling_0.3       lazyeval_0.2.1     munsell_0.4.3     
[41] crayon_1.3.4      

Start by reproducing the issue with the supplied rstan sampling() call, using four initialization lists and chains = 4. Trace how the init argument is assigned to chains and compare the resulting trace data; done means each chain uses its corresponding initialization rather than omitting the first and duplicating the fourth.