Cannot select destination pull request head

Issue:

Cannot create pull request against the correct destination branch.

Issue description:

I have a pull request (
https://bitbucket.org/MatthewTurk/yt/pull-request/11/misc-fixes-fixes-and-tests-for-operations/diff)\\
that I am unable to apply against the correct head on the destination
repository. I am trying to pull
request into the unitrefactor bookmark on the destination (when I opened the pull request, this was 943c100, since the PR has been merged manually now this changeset is no longer the head). When I view the pull request using the "Edit"
button, I am able to choose the correct destination branch and the diff
renders correctly.

Unfortunately, when I actually update the pull request, it appears as if
the diff is against the Stable branch - a completely different head
compared to the one I requested.

The strange this is, I was able to select the correct destination just a
couple of days ago. However starting yesterday this appears to not work
anymore.

This is for a mercurial repository. I have no idea if this problem is isolated to mercurial or is a general issue with bitbucket's branch pull request functionality.

Intended behavior:

I would like to be able to open pull requests against arbitrary heads in
the destination repository.

Actual behavior:

I am currently only able to open pull requests against the "default" head.

For now we are reviewing contributions manually on this repository,
obviously this is less than ideal. Please let us know if you discover
anything or need more information.