the lemmy backend was supposed to mark posts in NSFW communities as NSFW automatically going forward, which is why the UI option in standard lemmy-ui was removed to mark a post as NSFW if the community is NSFW.
unfortunately while the UI change made it to 0.19, the backend change didn't, so currently people posting in NSFW communities from 0.19.11 lemmy-ui will not have a choice to mark their posts NSFW.
looks like the federation issues were just related to feddit.nl sending to lemmy.world, did you figure out what caused this?