Okay this was extremely unexpected.
After reading cssbundling-rails’ (which I don’t use in my project) documentation, I found this: GitHub - rails/cssbundling-rails: Bundle and process CSS in Rails with Tailwind, PostCSS, and Sass via Node.js.
You must have
app/assets/buildsavailable. Add the directory with a.gitkeepfile, and you’ll ensure it’s available in production.
And that was it – the assets were digested correctly.
This is a bit perplexing because in the Fly production VM, I could find the assets compiled by dartsass-rails in that same exact folder (app/assets/builds/*.css), so I knew the folder existed; but the digested application-7577900c40f86606a5c6acaa00076c57a63f48a6.css etc were not in public/assets, so I also knew propshaft was not finding anything in app/assets/builds. There must be a dir creation order issue somewhere between dartsass-rails and propshaft.
Anyway, someone ought to add that warning to propshaft’s documentation I guess.