Possibly related: Build failures on every push except pass on cache rebuild - #5 by ec_symend
However I’ve been frequently deleting node_modules and yarn.lock and reinstalling, locally, before committing. So I don’t think my yarn.lock is the problem.
EDIT: Also possibly related, a very recent issue on Create React App (which I am using for this project): Babel fails on Cannot find module '@babel/helper-call-delegate' · Issue #8751 · facebook/create-react-app · GitHub