只有当我在循环的末尾放置一条print语句时，Foreach才会给出预期的输出

我使用foreach来并行化我的代码.然而，我发现无论我是否在循环末尾添加print语句，结果都是不同的.

下面是print语句的结尾(我得到了我想要的结果):

library(foreach)

grid <- expand.grid(
  lambda=c(1,2),
  mtry=c(1),
  nodsize=c(1)
)

n.cores <- parallel::detectCores() - 1
#https://stackoverflow.com/a/16718078/1979665
my.cluster <- parallel::makeCluster(n.cores, outfile = "")
doParallel::registerDoParallel(cl = my.cluster)
params.all <- foreach (i = 1:nrow(grid), .combine = "rbind") %dopar% {
  params <- grid[i,]
  params$rmse <- "foo"
  print(params)
}
parallel::stopCluster(cl = my.cluster)
params.all

结果是:

  lambda mtry nodsize rmse
1      1    1       1  foo
2      2    1       1  foo

以下是结尾处的print句话:

library(foreach)

grid <- expand.grid(
  lambda=c(1,2),
  mtry=c(1),
  nodsize=c(1)
)

n.cores <- parallel::detectCores() - 1
#https://stackoverflow.com/a/16718078/1979665
my.cluster <- parallel::makeCluster(n.cores, outfile = "")
doParallel::registerDoParallel(cl = my.cluster)
params.all <- foreach (i = 1:nrow(grid), .combine = "rbind") %dopar% {
  params <- grid[i,]
  params$rmse <- "foo"
  # print(params) LINE COMMENTED
}
parallel::stopCluster(cl = my.cluster)
params.all

现在的结果是:

         [,1] 
result.1 "foo"
result.2 "foo"

这是不是很奇怪，还是很正常？