Hey, i am facing the same problem and spent a lot of time in research. I have the same architecture for my project which has frontend and backend.
Frontend is based on Nextjs with app dir and Backend based on Nodejs-express and mongodb.
Hi !
We finally split the folders to put one on Netlify or Vercel (frontend) and one on Render (backend). We just had to rebuild the communication between both, but it was better than taking three more days searching a solution XD
Sorry not being able to help further…
Nice day