@J0 @Yumiscott It looks like there are two situations:

If you haven't already attached a picture, then setting a content warning will also turn on the sensitive-media flag-thing if you attach a picture afterwards.

If you've already attached a picture *and then* you set up a content warning, then the sensitive-media flag-thing doesn't get turned on automatically, and you need to hit the 'NSFW' button that shows up.