On current Bamboo versions, if the option "When pull request is created" for the Automatic branch management is selected, Bamboo will create a plan branch for all pull request made in the repository even if this PR unrelated with the default branch selected at plan's repository.

The current concept is understandable but, it would be great if bamboo users have the option to only create plan branches on pull request only if the pull request is related to the default branch.