It seems like this issue might ultimately be due to this issue:
I think one of my dependencies stdlib is a monorepo, and I believe it also compiles to binary or web assembly or something, which the guide said might be a problem because it doesn’t get re-built. I’ve frequently had problems with it running correctly locally until I clear the yarn cache and reinstall. And although my memory is hazy, I think the Netlify pull request previews started failing around the same time I added this library to my project.