[racket] Custodian behavior on normal exit

I think I have a misunderstanding on the behavior of custodians. I'm trying
to write a Racket program that shuts down any subprocesses it may have
created when the program exits, regardless of whether the exit was graceful.

I would have thought that the root custodian would have taken care of this,
but it appears not. Consider the following program:

;; kill-test.rkt
#lang racket

(parameterize ([current-subprocess-custodian-mode 'kill])
  (subprocess #f #f #f (find-executable-path "racket") "sleep.rkt"))

;;(custodian-shutdown-all (current-custodian))

-----------

sleep.rkt is just a Racket program that sleeps for 5 minutes, then exits.
if I run kill-test.rkt as-is, it immediately exits, but sleep.rkt continues
to run for 5 minutes. However, if I uncomment the custodian-shutdown-all
line, the subprocess is killed immediately when kill-test exits. Also, if I
halt the program with Ctrl-C before it exits (say, if I inserted a sleep at
the end), the subprocess is killed without the manual call to
custodian-shutdown-all.

My assumption was that the root custodian would always call
custodian-shutdown-all on itself when a Racket process ends, regardless of
whether the process is shutting down gracefully or not. Is that not the
case?
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