@Gargron @chickamora @atomicpoet @Tinu

Its a bug with mastodon - if you reply to a post marked with a "content warning" then remove the text, but don't click the blue "CW" then type a bunch into the input box then add an image; the usual "Mark media as sensitive" checkbox is hidden below the input text and the scroll bar only scrolls to show the image. This means your only indication it will be marked sensitive is the easy-to-miss blue "CW".