Smart mirror failing to sync after merging PR

Set up an instance of Bitbucket Server which is a Smart Mirror of a repository on bitbucket.org

Merge a PR, making sure to have the option selected for "Close source branch".

Expected behaviour is that the updates are synced to the mirror. Actual behaviour is that they updates may not be synced (not even when the scheduled task runs to sync).

Workaround is to push a new commit to the repo (or tag the PR branch, to trigger a new commit), which does sync the latest changes to the mirror.