Merge shown as complete in UI but not committed to destination branch

We had an example of a merge from a feature branch to develop branch where the change was not actually reflected in develop after clicking the merge button.

We know this isn't an issue where someone force pushed develop in a different state because directly pushing to develop or master is prohibited in our repo.

Other developers confirmed that by all appearances in BitBucket UI the merge was completed correctly but the commit and merge commit never showed up in the destination branch.

This report differs from
BCLOUD-15399 because the merge was shown to be completed but the commit did not actually appear in the branch. Unlike other merges, the merge was shown as "<user> remotely merged the pull request from <feature branch> to develop" This merge was performed in the identical manner as other merges (BitBucket user interface) so it's unclear why the message varies in this case.

Priority marked as minor because it only happened once but is arguably major because this caused a bug on a production website.