We seem to be having trouble with a handful of the larger lemmy instances, as well as some misc other instances even though these instances aren’t dead-dead: https://fedia.io/federation (at bottom of page - “Dead Instances”).
I thought maybe it would’ve been a problem with a specific lemmy version, but there are several versions in the list and also some still-living Mastodon instances that are running reasonably up-to-date versions.
I just looked at these 3 on my PieFed server to see if maybe it’s also seeing a similar issue, but all 3 mentioned Lemmy instances are federating fine and have all been seen within the past second here.
Weird, I know that I saw one comment from ani.social come in very delayed (like a couple of days) despite showing up on both the host [remote] instance and Mastodon.