When creating a multi-branch, even when selecting the "Bitbucket Server trigger scan after push" checkbox, after saving the job, a webhook doesn't get created in Bitbucket, and hence subsequent pushes don't trigger a branch re-index of the job. After opening the job's config, we can see the BB trigger checkbox is selected, and after re-saving the config (without changing anything), a webhook is created in Bitbucket this time.