I believe there was a change to some Lemmy communities where if the community itself is marked as "NSFW" there's no ability to toggle it off anymore: https://github.com/LemmyNet/lemmy-ui/pull/2887 (https://join-lemmy.org/news/2025-04-08_-_Lemmy_Release_v0.19.11).
If the post isn't flagging automatically as NSFW in a NSFW community, then there may be a backend bug that needs to be reported to the Lemmy developers via GitHub.