`setorder()` on data.frame input order only the sort column

Start by reproducing the minimal example through data.table::setDT() and data.table::setorder(), then inspect how the data.frame columns are shared and reordered. The report names no repository files or tests; done means establishing and correcting the inconsistent column state while preserving the correctly sorted result.

I'm passing a data.frame to a function that sort it using data.table::setorder. Before sorting, I'm calling data.table::setDT on the input (in actuality I need to later call some specific data.table functions on the input). This is the function I'm using:

test_order <- function(x) {
  data.table::setDT(x)
  data.table::setorder(x, score)
}

and this is how I'm calling it:

x0 <- data.frame(id = 1:3, score = c(20, 10, 30))
x1 <- test_order(x0)

The results seem strange:

• The type of x0 is changed to a data.table - that seems expected;
• The column x0$id is not modified - it's address (using data.table::address) remains the same, and it also remains in the original order;
• The column x0$score also remains at the same address, but it is now sorted, so it's out of sync with the id column;
• The resulting x1 has the rows sorted correctly; also, the score column is at the same address as the score column of x0, but the id column is, of course, at a different address. Here is what I get:

Minimal reproducible example

test_order <- function(x) {
  data.table::setDT(x)
  data.table::setorder(x, score)
}
> x0 <- data.frame(id = 1:3, score = c(20, 10, 30))
> x0
  id score
1  1    20
2  2    10
3  3    30
> x1 <- test_order(x0)
> x0 # note that the id column is in the original order, but the score column has been re-ordered
   id score
1:  1    10
2:  2    20
3:  3    30
> x1
   id score
1:  2    10
2:  1    20
3:  3    30

Output of sessionInfo()

> sessionInfo( )
R version 4.0.5 (2021-03-31)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Debian GNU/Linux bookworm/sid

Matrix products: default
BLAS:   /usr/lib/x86_64-linux-gnu/openblas-pthread/libblas.so.3
LAPACK: /usr/lib/x86_64-linux-gnu/openblas-pthread/libopenblasp-r0.3.20.so

locale:
 [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C              
 [3] LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8    
 [5] LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8   
 [7] LC_PAPER=en_US.UTF-8       LC_NAME=C                 
 [9] LC_ADDRESS=C               LC_TELEPHONE=C            
[11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C       

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

loaded via a namespace (and not attached):
[1] compiler_4.0.5    data.table_1.14.2