Merge was being prevented, and I thought that this may be due to the issue in Jenkins itself. After it's been resolved, forced builds succeeded, yet merge was still not enabled.
From this thread I gathered that the build count is attached to commits, not to the PR itself. So I have made a trivial change and committed it forcing a new build which was successful. The bitbucket PR page shows it as such, but yet the merge button still says "There is 1 issue preventing you from merging this pull request. You need a minimum of one successful build before this pull request can be merged."
I am puzzled. Any clues?