When I create a branch from a ticket in JIRA it gives me the option to choose what branch to base the new branch off of. We follow the workflow where normally you're branching from a branch and then merging back into that branch. This applies even for cherry picks where you're moving from one master branch to another (or from master to a release branch or something like that.)

In any case, since I've already specified where the branch should originate; and the typical workflow is to merge back into that branch, it's unexpected that bamboo / bitbucket would choose to create the PR to go back into the default branch.

My suggestion is to have the PR by default merge back into its immediate parent; and maybe, if that differs from the default branch to put a ! warning, or a model dialog asking if that's what you want to do.