As always with these things, I tried a bunch of stuff and I'm not sure exactly what fixed it, but my last deploy after pushing to git succeeded. Here's what I tried:

• I noticed an earlier deploy had no "index.html" in the /output folder of my deployment, whereas the successful deployment (from the command line) did have a index.html file in there.
• I moved the index.html into the /public folder of my project.
• build fails and the logs say that the package.json can't be found. It no longer succeeds and gives a 404 error on the homepage.
• I run an NPM install.
• build fails
• I change the settings for the root directory to "." and also override the build command to "npm run build" to match build logs I got from a successful build from the command-line.
• build fails
• I get rid of the overrides (so I'm just using the default deployment settings now). And move index.html back to the root directory (this is where it has always lived in my project). I then, on a whim, decide to track the "package.json" in git (it was previously in gitignore).
• build succeeds.

My conclusion is that an NPM install, and ensuring you are tracking the package.json in Git may help resolve the 404 error. Cheers.

I'm glad you got it working! Thanks for sharing what you did 😄