I don't have a link handy, but I saw this reported before and when someone went to lemmynsfw to check the posts, they actually had been properly marked as nsfw.

My unsubstantiated theory is that federation is working pretty crappily right now and so lots of federation messages get dropped. When a new post or comment gets dropped you don't notice because... well... you don't see anything. But when a post makes it through and an edit gets dropped, that can be more visible.

So likely what's happening is that on some small percent but medium absolute number of lemmynsfw posts, someone makes an honest mistake and fails to tag it. They either notice themselves and fix it, or a mod asks them to and they comply... so on lemmynsfw it looks right. But the federation message with the edit gets dropped by your instance, and for you and others viewing from there it forever remains sfw in error, even though all the right steps were taken. I've also heard mods discussing this happening with post removal moderation actions, the post gets removed on the community's home instance, but the federation message containing the removal gets dropped by some big instance and the post blows up there anyway where the mods are actually unable to shut it down.

I don't think there's anything anyone can do about this other than maybe the lemmynsfw admins tuning their federation worker counts (though they may have already and that may no longer be needed in recent Lemmy versions), or the Lemmy devs working on federation scalability so a larger percentages of federation messages get reliably delivered.