Looks like this was an issue of juggling multiple instances of React in an npm monorepo. Even though these all had distinct package.json files, having different versions of React in the common node_modules folder was a timebomb. I'll need to clean out the project further, and maybe get all the projects on the same React version. In the interim, a co-worker found that using --legacy-peer-deps was enough to get everything at least working, ie: npm install --legacy-peer-deps -w