`cabal build` does not rebuild `source-repository` deps that changed upstream

I observed the following:
1. In `cabal.project`, depend on an upstream git repo, with `tag` omitted (so, depend on the `master` branch).
2. Call `cabal build`. I my case this failed because an upstream type error.
3. Push a commit to upstream `master` (that fixes the type error).
4. Call `cabal build` again. It fails with the same error.

So, it seems that cabal's caching logic does not take upstream changes into account if the `tag` didn't change.
Modifying the `cabal.project` file with putting in `tag: ` then triggered the rebuild (and succeeded).