npm script hangs on Windows with "Terminate batch job (Y/N)?" prompt

[DeltaWhy]

My Procfile contains a line webpack: cd gui && npm run watch, which fails to exit cleanly on interrupt (CTRL-C). Instead I get a prompt Terminate batch job (Y/N)? which I can't answer.

When executing npm run watch from a cmd promt, I can suppress this prompt by redirecting input from NUL (Windows equivalent of /dev/null), however doing the same in my Procfile doesn't solve the issue - there's no prompt printed but it still hangs. Using yarn instead of npm does exit cleanly, with or without the redirect.

Any idea what's going on?