react-native bundle never quits, metro-config implicated

I have a react-native monorepo which no longer builds bundles. I can run in development just fine, but when I try to build a bundle like so:

react-native bundle --platform ios --entry-file index.js --dev false --bundle-output ./ios/main.jsbundle --assets-dest ./ios --sourcemap-output ./sourcemap.js

It just hangs after this:

                    Welcome to Metro!
              Fast - Scalable - Integrated


info Writing bundle output to:, ./ios/main.jsbundle
info Writing sourcemap output to:, ./sourcemap.js
info Done writing bundle output
info Done writing sourcemap output
info Copying 118 asset files
info Done copying assets

When I use why-is-node-running, I see these handles:

# TTYWRAP
/app/node_modules/metro/node_modules/debug/src/node.js:68 - 2 === fd ? process.stderr :

# SIGNALWRAP
/app/node_modules/metro/node_modules/debug/src/node.js:68 - 2 === fd ? process.stderr :

# TTYWRAP
/app/node_modules/metro-config/src/defaults/index.js:187 - reporter: new TerminalReporter(new Terminal(process.stdout))
/app/node_modules/metro-config/src/defaults/index.js:198 - return getDefaultValues(rootPath);
/app/node_modules/metro-config/src/defaults/index.js:14  - var info = gen[key](arg);
/app/node_modules/metro-config/src/defaults/index.js:36  - asyncGeneratorStep(gen, resolve, reject, _next, _throw, "next", value);

I don't know that I trust the line numbers exactly, but it is super weird. metro-config 0.64.0, metro 0.64.0, RN 0.64.0-rc.3. Fails both with and without hermes. Monorepos are notorious for issues with RN, so I do suspect that, but it was working on RN 0.63 and related metro setup.

Reproduce the shown react-native bundle command with Metro 0.64.0 in a monorepo, then inspect the reported handles and metro-config/src/defaults/index.js around the TerminalReporter setup. Compare the behavior with the previously working RN 0.63 setup; done means the bundle, sourcemap, and assets are written and the command exits instead of hanging.