updateBranch seems to be not working

[tanvn]

Hi, thank you for developing an amazing app.
I am trying your app with one of my public repository.
I am very happy to see that some of my PRs have been merged automatically.
And I tried to verify the setting of updateBranch: true
Here is my config file : https://github.com/tanvn/fp-in-scala-with-ci/blob/master/.github/auto-merge.yml
And here is my PR : tanvn/fp-in-scala-with-ci#22

I saw that the update branch is visible but the bot did not update the branch automatically.
I have to update the branch manually and after that the bot showed up and merged the branch for me!
The branch of the PR is in the same repository so I expected the bot to update the branch for me.
Am I missing something ?