Only some comments are being federated

Lemmy.ca currently shows 22 comments on this post, but its native instance, lemmy.ml, shows 35 comments.

It's not (or not only) a matter of only older comments being present, or of comments being from different instances: for example, this comment from a lemmy.ml user doesn't appear on lemmy.ca but this later comment from a different lemmy.ml user does.

Is this a known problem?