Currently the documentation for automatic branch merge does not mention that the default branch will also be part of the merge. All it stays is that release branches will be considered.

My test setup:
develop is my default branch, three release branches that branched out from develop. I create a bugfix branch from develop, make a change, and generate a Pull Request from my bugfix branch to release/1.0. Based on current documentation the target branches for my merge would be release/1.0, release/1.1 and release/1.2, and not include my default. The screenshot shows the actual result were the default branch is included.