Well, it looks like the SDF instance is catching up: suddenly a whole bunch of comments, and all the upvotes from the lemmy.ml instance now show up in the SDF view. Too many upvotes in fact 🙂 I guess the downvotes are still getting processed.
Maybe this is all sorting itself out, whatever the bottleneck was...
Oh and by the way, I wasn't implying that SDF is responsible for this. It was just a general comment on the state of federation in Lemmy, which - the more I look at it - seems really quite fragile.
I see exact same amount of comments from my instance as from lemmy.ml, so I'd guess it's probably something specific to sdf.org. Maybe federation queue?