renaming a branch in mercurial breaks editing open pull requests

TL;DR: if you rename a mercurial branch, open pull requests issued to the original name get dysfunctional

How to recreate:

1. On a given mercurial repository, create a branch, add a commit, push it to bitbucket
2. Open a pull request on that branch
3. Rename the branch as follows:

1. 1. Create a new branch from the last commit of the existing branch, like "hg branch new_branch"
   2. Create an 'empty' commit by touch-ing any of the contained files and commiting, so that the new branch exists
   3. hg update to the old branch name
   4. hg commit --close-branch to close the old branch
2. Push to bitbucket
3. Open the existing pull request. Click on Edit in the top right corner.

What should happen:

• I would like to select the new name here and pull in the ('empty' commits to update the pull request

What actually happens:

• A rotating progress indicator spins on the bottom of the screen.
• I cannot select any originating branch name using the pull-down menu or typing (see attached screenshot)