Pull request allow specification of destination branch, which is ignored (BB-2125)
In order to reproduce this issue, create a repository, create default and develop branches, and then folk the repository. Once forked, work on a new feature branch in the clone, and then send a pull request from the new branch, specifying develop as the destination branch.
Although you ask asked for a commit message, inspection of the parent repository will reveal that the merge happened on the default branch, regardless of the fact you have specified that develop is the destination branch.
If you are using named branches, this means that it is not possible to use this feature.