Steps to reproduce

1. create application link between Bamboo v5.7.2 and Stash v3.6.0
2. create a repository in Stash
3. commit something to the repository
4. create a Project / Plan in Bamboo
5. set up Stash repository in "Plan configuration >> Repositories" tab
6. under "Plan configuration >> Triggers" tab, select "Stash repository triggers the build when changes are committed" as "Trigger type"
7. under "Plan configuration >> Branches" tab, enable "Create plan branches for branches detected in the repository"
8. run your build
9. disable your build Plan
10. create a new Branch

    git checkout -b newbranch
    

11. commit something to the "newbranch"

You will notice a new branch will be displayed under the Plan in Bamboo and the same will trigger a build.