Fork sync deletes branches which are in a pull request

Scenario:

1. Create a pull request from a branch in a forked repository back to the main repository
2. In a local checkout, fetch the branch from the forked repository and push it to the main repository
3. Delete the branch from the main repository, but leave the branch in the fork

Expected result:
The pull request is unaffected, because neither the source branch (in the fork) nor the destination branch (in the main repository) were changed

Actual result:
Stash declines the pull request when the branch in the main repository is deleted. The pull request cannot be re-opened because stash believes the source branch has been deleted.

Workaround:
Push the branch to the local repository, and delete it once the pull request has been merged.