Preview/branch deploy not working for functions

we have a pipeline in bitbucket that runs yarn install, netlify build and netlify deploy --alias=. We set NETLIFY_AUTH_TOKEN and NETLIFY_SITE_ID before the build.
netlify.toml is like
command =
functions =
publish =

According to the build output, every this is fine…
- Hashing files...

✔ Finished hashing 305 files and 19 functions

- CDN diffing files...

✔ CDN requesting 11 files and 6 functions

- Uploading 17 files

✔ Finished uploading 17 assets

- Waiting for deploy to go live...

✔ Deploy is live!

But, the functions are not accessible. There is just an empty page. No issue, no error.

can you tell us your site name please so we can take a look? thanks!

this one for example
compared to