In https://github.com/flatpak/flatpak-github-actions/blob/master/flatpak-builder/index.js#L59, there's this:

    this.branch = 'master'

which sadly overrides the branch name from https://github.com/flatpak/flatpak-github-actions/blob/master/flatpak-builder/index.js#L29

Sadly this caused OBS to publish a beta release in the master branch of Flathub Beta, instead of beta.