Best practice: Syncing the master and beta branches

I am wondering how to cleanly synchronize the master and beta branches of my flathub repositories. A typical workflow is that beta is updated more frequently than master, but that after an update to master, beta should be identical to it until new beta releases are attempted. In such cases I’d like beta and master to point to exactly the same commit SHA. But I haven’t found a way to do this. I can create a PR from master to beta, but with GitHub’s inability to do proper fast-forward merges, it always creates either a merge commit or a modified commit in the beta branch that make the branches point to different SHAs, even though the content is identical.

I have tried creating a PR, waiting for the build to finish and succeed, and then pushing the same head commit to beta, but it’s forbidden, too.

Am I missing something? Is there a trick to achieve this?