Process sentinels don't run during init in GUI Emacs

Question

Ok, this issue I'm running into seems pretty weird. I'm running an async process during init and I have set a process sentinel to tell me when it exits. This works fine in terminal Emacs but in GUI Emacs the process sentinel never runs.

Here is a minimal working example:

(setq my/test-flag nil)
(defun my/test-sentinel (proc &optional _change)
  (setq my/test-flag t))
(let* ((name "test")
       (buf (generate-new-buffer (concat "*" name "*")))
       (proc (let ((process-connection-type nil))
               (apply #'start-process name buf "sleep" '("3")))))
  (set-process-sentinel proc #'my/test-sentinel)
  (while (not my/test-flag)
    (sleep-for 0.05)))

If you put this in init.el and run terminal Emacs, it will hang for 3 seconds as expected before continuing on. GUI Emacs, on the other hand, will hang forever, indicating that my/test-sentinel is never run, even though the process does finish after 3 seconds.

Background info:

• I am using Emacs 27.1 from the Arch Linux repositories.
• I failed to reproduce this on macOS using the emacs Cask.
• Replacing sleep-for with accept-process-output gives the same result.
• If the snippet is run by the user after init, the sentinel runs as expected in GUI Emacs.
• This issue arose while debugging use of async-wait from the async package during init.