possible sort bug?

``` r
dt = data.table(id=1:30, nn = paste0('A', 1:30))
smp = sample(30, size =10)

#  data is sorted as expected 
aa = dt$id %in% smp
dt[aa, ]

# unordered result
dt[id %in% smp, ]
```

Is this a bug or by design?
