Neither of my posts from yesterday and today have made it out to their communities' home instance (lemmy.world), and neither has received any comments or votes. It seems like something to do with outgoing federation is stuck.
Edit to clarify:
The problem I'm seeing affects posts. I don't know if it affects comments.
It's possible that it only affects lemmy.world communities, but it definitely affects them even on other instances. Neither of these posts are showing up on any remote instance that I've seen:
https://lemmy.sdf.org/post/21620419
https://lemmy.sdf.org/post/21673588
6 days later, it's happening again:
From experience sh.itjust.works sometimes has this problem as well.
They also have one thing in common: Cloudflare
Perhaps it's getting "protected" from federation considered a DoS.