Edit: After reading your comments, and testing some more, I must say that I've misunderstood how it all works.
I should've thought of Mastodon users like separate Lemmy communities...but not exactly. What confused me is the fact that you could look up a profile on a remote instance and see their posts, but they would be very delayed. On Lemmy, if your instance hasn't "discovered" a community, you wouldn't see it at all.
I followed a random user (whos posts were last synced many days ago), and it started syncing normally (it took ~1h for it to start, but it seems like it worked and now it's syncing their posts "in real time").
~~By accident I noticed that one instance had more japanese posts in the all feed than the other one. I thought maybe the other instance has certain languages filtered or they might be defederated from certain instances, but neither was the case. I found out that the other instance just fetches the posts from other instances much slower (days).~~
~~Then I decided to open 10+ (popular to fairly popular) instances and compare how quickly or slowly they sync with each other.~~
~~It's really bad and really random. Some instances sync perfectly with each other, some take hours, some take days, some take months...
I do not use Mastodon but if I did, finding that out would just make me not want to use it.~~
~~It reminds me of that time when there was a bug in Lemmy which made the federation broken, and that was very annoying, but we knew that there was a bug and that it was being worked on, and it was fixed fairly quickly.~~
~~But on Mastodon, from what I've seen, it doesn't even depend on the version the server is running, it truly just seems random.~~
~~It just seems odd to me that Mastodon (more popular and older software than Lemmy) would have such a glaring issue.~~
~~Wouldn't that be the first priority of every federated platform? For federation to work properly, because if it doesn't, then it can't compete with the centralized ones at all.~~
Yeah there are multiple ignored GitHub issues about Mastodon's federation of replies, going back many years. It's never getting fixed. This realisation sent me on a multi-week quest to find a platform that does replies properly. Akkoma and Friendica seem better at replies but have other shortcomings.
It cannot be "fixed" unless you centralize into a single firehose like Xitter and Bluesky.
Those of us running smaller instances can choose to use relays and fetcher-tasks - and there is a PR to put one such fetcher into Mastodon - but if the goal is 100% of everybody always sees 100% of the content then no decentralized solution will ever offer that.
It can be, it just involves contacting the site that hosts the top-level post and having it forward the replies itself. It'd be a change to the distribution model, and not a simple one, but can absolutely be done.
That's just replies to a post. It doesn't solve not all posts reaching everybody.
I wouldn't be quite so pessimistic. There's a commit for #FetchAllReplies (by @jonny@neuromatch.social, I believe) that seems to be shaping up well, with a seemingly healthy debate going on. Just yesterday @Gargron@mastodon.social posted in agreement that it is a must-fix issue.
They're moving slow, but their reasons for doing so doesn't seem to boil down to an unwillingness to fix it.
Excellent news!