Why unstable_cache fetches twice every build on Vercel?

[roxizhauk]

Repo: https://github.com/roxizhauk/test-unstable-cache

The data cached by unstable_cache is updated every build on Vercel, querying to my database on PlanetScale TWICE (always). Is this an expected behavior? And if so, how do I modify it so that it acts exactly the same as Data Cache, meaning it will be persistently cached across deployments fetching once in the initial build?

This getWords() is used in a static page ("/").

The following log shows how the fetch works on development.

In local build, it still fetches twice in the initial build but the cache seems not updated in another build (becasue the query wouldn't be fired in PlanetScale).

[roxizhauk]

The same function used in a route handler only fetches once every build.