GitHub Action not being triggered for contributor

I am a contributor of GitHub - FirebirdSQL/firebird: Firebird server, client and tools and we have GitHub Actions set up.

When I create another branches and PRs for that project, it does not trigger a build. But the action has no filter for branches.

For example, see Introduce the external memory pool by asfernandes · Pull Request #6991 · FirebirdSQL/firebird · GitHub. Only AppVeyor has been run.

When user’s create PRs from their own forks, the action is run.

What does may be causing that issue?