It's worth noting that federation only kicks in AFTER someone subscribes to a community,.and only for new comments and posts on that community. Is it possible that you're looking at several-day old posts, where someone from lemmy.world subbed last week and so they're fully filled in... But someone on feddit.uk only subbed yesterday and so you only see comments from the last day or so but are missing the ones before that?

If that's what's going on, it happens on all instances (though obviously on bigger/older ones the odds are better that someone else subbed before you and you won't notice), and it self resolves. A few days from now, you'll mostly be looking at posts that have been made since you subbed and you'll get a complete view of them. For old posts, click the fedi-url icon to visit them on their home server... you'll get a complete view but won't be logged in to vote/comment.