See the following example. The environment variable COLUMNS can not be update correctly when the terminal window size is changed.
R> Sys.sleep(5) # I changed the window size during this period of 10 seconds.
But if I exit from the R session, I can see COLUMNS is changed in the bash shell from which R was called.
~$ echo $COLUMNS
That is because the shell usually manages it. E.g., on OS X, neither 'vi', 'emacs', 'less', or 'more' have COLUMNS in their environment, at least not in a user-accessible way. (Try :!env, M-x shell-command env, !env, !env, respectively).
It seems that an application program could potentially catch SIGWINCH and adjust settings accordingly, but I don't see a compelling use case for R to do this, and it may not be easy to get right. At any rate, it is not reasonable to count this as a bug in R.