I appears that version 0.28.6 introduced a bug related to the file watching mechanism. When I alter a file I get the following error message:

ModuleNotFoundError: Module not found: Error: Can't resolve '/ncc-bug/node_modules/@vercel/ncc/dist/ncc/loaders/stringify-loader.js' in '/ncc-bug'

I've created a minimal reproduction of my setup here: https://github.com/kilahm/vercel-0.28.6-stringify-loader

This setup works (with the file watching) for @vercel/ncc@0.28.5, so it seems like it was introduced by an update to either ts-loader or webpack.