Thank you tyf! I luckily found the solution to my particular issue - sharing here in case others encounter it.
I used the Next-Flask-Starter template, which routes calls to the Flask backend via a root-level api/ directory. Vercel didn’t like that there was this root-level api/ directory and then also the app/api/auth/… directory for auth0. As soon as I changed the Flask api/ route to something else, everything worked swimmingly.