The /rest/branch-utils/1.0/projects/{projectKey}/repos/{repositorySlug}/branches/info/{changesetId} REST endpoint documentation states the following:

Gets the branch information associated with a single changeset from a given repository.

The documentation should be updated to include clarification that once a commit is on master (or whatever the default branch is) Stash considers it "merged" and will only show this commit as existing on master. Without this clarification, it's easy to think that all branches that this commit exists on should be returned regardless if it exists on master.