I have run across two or three instances of federated content from LemmyNSFW not correctly showing up as being tagged nsfw when viewed from Lemmy.world. The one time I actually did some investigation, I found the post was correctly tagged as nsfw when browsing LemmyNSFW directly.

My assumption was that the post might have been created without the tag, but was edited to include the tag later, but that the edit may not have replicated across federation. Maybe if no other content is changed it doesn't correctly populate the edited tag? Just a guess.