App.css not found in prod

I’m sure I’m just missing something obvious here, but in dev my app.css compiles nicely enough and everything looks nice. In production, the link to (now, in prod) app-gibberish.css?vsn=d just gives me a 404 and so clearly there is no formatting. What step am I missing in the build process that would be pulling this in correctly? My css all builds from web/static/css into app.css using brunch, and I make a release with the following:

mix release.clean
MIX_ENV=prod mix phoenix.digest
MIX_ENV=prod mix compile
MIX_ENV=prod mix release


As the docs says, you need to run brunch build --production before running mix phoenix.digest