Heroku Build Not Copying React Files - node.js

Question

I have a Node application, including an express server, set to respond to various backend requests and then to try to serve static content from a React app's build folder. It works great locally. The two different package.json files in the server and client respectively build the React app, copy its build files into the server's ./dist folder, then builds the server app and does the same, then runs the server. Everything is perfect.
However, on Heroku, the React build folder is never copied into the server's dist folder, and I get 404 errors trying to load the app. I know this worked at some point but I'm not sure what I changed that broke it and nothing is making sense to me to get it working again.
"scripts" section of the client (react) package.json:
"scripts": {
"start": "react-scripts start",
"react-build": "react-scripts build",
"build": "npm run clean && npm run react-build && npm run copy && cd server && npm run client-build",
"clean": "rimraf server/dist/",
"copy": "copyfiles build/**/* server/dist -E",
"test": "react-scripts test",
"eject": "react-scripts eject",
"prod": "cd server && npm run start"
},
and the "scripts" section of the server's package.json:
"scripts": {
"test": "echo \"Error: no test specified\" && exit 1",
"dev": "nodemon -i dist -i creds index.ts",
"clean": "rimraf dist/",
"copy-files": "copyfiles views/**/* assets/**/* creds/**/* dist/",
"copy-web": "copyfiles -u 1 ../build/**/* dist/ -E",
"build": "cd .. && npm run react-build && cd server && npm run clean && tsc && npm run copy-files && npm run copy-web",
"client-build": "npm i && tsc && npm run copy-files",
"start": "node dist/index.js"
},
Heroku, of course, runs npm run build from the root folder to kick off the build process, which is the client's package.json, so it runs:
npm run clean && npm run react-build && npm run copy && cd server && npm run client-build
Specifically the npm run copy doesn't seem to be doing anything. When I hit the ls through the heroku-cli, the folder is not there. I'm not getting any errors in the build.
I have been able to put a cludgey piece of code into the main server so that it checks to see if the build folder exists where it's supposed to exist before setting up the static middleware, and to go up the file tree to where they're supposed to be copied from if it doesn't, so it's working, but I don't think I should have to do this and I can't just point to the initial React build location or I'd be serving stale build's while I'm in dev locally instead of a specific dev page that links to the webpack npm run start instance that updates as I hack at it.
I understand that between the workaround in prod and just not relying on the server's static files in dev is "good enough" for me to move on, but my OCD won't let me until I understand why copyfiles works fine copying the server's extra files into dist but can't manage to copy the React app to the place it's supposed to go.
This seems like a pretty basic thing, and long run my plan here is to serve multiple independent React applications as "widgets" from different paths on the server and if I can't get the root path to behave because I'm an idiot then idk what kinds of problems I'm going to run into down the line.
TIA, lmk if any additional info is needed.
CORRECTION: I had been building an outdated version without the -E flag on the copyfiles call. Once I updated that, I did in fact get an error that no files were copied. To reiterate though, the files ARE in the build folder where they were supposed to be copied from, just not in the dist/build folder where they're supposed to go, as confirmed now by copyfiles deciding it had nothing to copy.
Is this an async issue? I thought the && suggested that the left side (the build) had to finish before the right side (the copy) would execute?