Solved. I made sure to add the future flags found in https://remix.run/docs/en/2.13.1/start/future-flags#v3_singlefetch, of 'v3_singlefetch. This is used as .json() functionality is deprecated in remix.

All you have to do is return the values (in Remix), no need to json.stringify them or put them in a Response. So change return Response.json({ error: 'Invalid Form Data', form: action }, { status: 400 }) to return {error: 'Invalid Form Data', form: action}