When using buildifier -r, files ending in the .BUILD suffix do not get included in the checked set of Starlark files. However, this is a somewhat common naming scheme for files in third_party/ folders; see for instance six.BUILD in rules_cc/third_party.

Looking at isStarlarkFile() in utils.go, it looks like this would be the place to add to the list of accepted extensions. Is there some reason it's not already there?