A user on my team properly committed a file that is tracked/trackable by LFS.
He then pushed the change to his origin. The LFS file appears there as expected,
in LFS file storage.
The user then issued a pull request to our upstream repo. The PR was approved and merged. However, the LFS file was not transferred to upstream. The merged commit, when viewed from upstream has an entry for that file, but the preview says "LFS file missing."
Bitbucket does clearly say here that you must manually take care of LFS files while forking, but the mention of pull requests is much more subtle, which is surprising since the pull request use case is so much more common.
In any case, if it can't handle pull requests, the LFS feature is broken. Pull requests (and forking, for that matter) are important parts of the git development process, and leaving a repository corrupted (the repository cannot be pulled properly until the files are properly in LFS storage) is a serious bug. Until it is fixed I am discouraging frequent LFS use.