When cloning/pulling via SSH from a mirror, all requests for LFS files get redirected to the Upstream rather than simply converting to an HTTP(s) URL of the Mirror itself. HTTP(s) on the other hand is not affected and successfully retrieves all content from the mirror without the need to contact the upstream.
Add a Mirror to your DC instance and sync a repo with LFS files. Attempt to clone the full repo from the mirror with "CURL_VERBOSE=1" and you will see the connection to the upstream
All content, LFS included, should be downloaded from the mirror.
Regular Git content is retrieved from the Mirror but once complete, LFS files are then retrieved from upstream.
There is no workaround for SSH but you can use HTTP instead to retrieve all files from the Mirror.