Not a bug but a feature of how the system works. That post came out years before piefed.world was a thing. New instances don't auto ingest everything from every instance they are peered with that was ever posted.

Chances are, you may have been the first person to visit that post from PW. Doing that synchronized the basic post but not all the comments. At least not immediately. I'm not clear if it will do it later now that you've accessed it. But typically an instance doesn't synchronize content its users aren't accessing. Otherwise all instances would quickly balloon up to a size unmanageable by most groups.