API routes returning 404 — functions work via "Invoke Function" but fail via HTTP - Help

Title: Serverless functions return 404 via public HTTP, but work fine via "Invoke Function" — across two separate projects/repos

Hi everyone,

I'm dealing with a persistent issue on one specific project in my team, and I've run out of ideas after extensive troubleshooting. Hoping someone has seen this before.

The problem

All serverless functions inside api/*.js compile successfully, show up correctly in the deployment's "Output" tab, and execute correctly when triggered via the "Invoke Function" button in the dashboard. However, when hitting those same routes via normal public HTTP (browser, fetch, curl — anything outside the dashboard), every single one returns:

404: NOT_FOUND
Code: NOT_FOUND

This affects all functions in the project (chat.js, db.js, export.js, guia.js, keep-alive.js) — not just one.

Environment

Team: LumenLabs (Hobby plan)
Region: gru1 (São Paulo)
Function syntax: standard Node.js runtime handler — export default async function handler(req, res) { ... res.status(...).json(...) }
Repo: GitHub, main branch

What I've already ruled out

Code/syntax — validated, no errors; functions run fine via "Invoke Function"
vercel.json — tested both the legacy format (builds + routes, with explicit /api/x → /api/x.js mappings) and the modern format (rewrites, no builds). Both formats are identical to what two sibling projects in the same team are using successfully right now.
Deployment Protection — Vercel Authentication and Password Protection both disabled
Firewall — active, no custom rules, all traffic logged as "Allowed"
Domain — removed and re-added the alias domain; tested both the alias and the deployment-specific auto-generated domain directly — same 404 on both
Cache — tested in incognito, and redeployed with "Use existing Build Cache" unchecked
Recreated the entire project from scratch — deleted the original project and created a new one from the same GitHub repo, with fresh env vars — same result
Created a brand new, independent repo — copied the exact same files into a second GitHub repo, connected as a new project in the same team — same 404
Reverted a recent code change to an earlier working version of one function, to rule out an app-level regression — no change in behavior

What does work, for comparison

The static index.html in the same project loads fine
The exact same route patterns (/api/chat, /api/db) work perfectly on two sibling projects in the same team, with equivalent vercel.json and function syntax
"Invoke Function" in the dashboard always succeeds

Given functions build, appear in Output, and execute correctly when invoked directly — but the public HTTP routing never reaches them, regardless of domain, vercel.json format, or even a fully recreated project — this really looks like something broken at the platform/routing level tied to this specific project (or its history), not the application code.

I already opened a support conversation and got confirmation that the function syntax is correct and standard. Posting here too in case anyone has hit something similar, or has ideas I haven't tried yet.

Sample Request IDs (for anyone from the Vercel team who wants to trace this):

gru1::dh57q-1783802334436-338080cdbfac
gru1::wt5rd-1783797646538-2024ddd8b450
gru1::zkbsf-1783795581502-f6f03c536b8c

Thanks in advance for any pointers!