You can see it highlighted in orange here next to the moon icon (the moon is for "unlisted" visibility).

I think it represents an alarm light 🚨.

In this case I created a new reply and left the same CW as the original post, which is the default behaviour.

Maybe it depends on the client. On elk.zone I can toggle the content warning icon (🚨) to disable it in the reply

I just noticed that the style for the tangent block's title is a little bit off on Android Firefox and thought you may want to know since you clearly put effort in expressive styling (and I still refer to the CSS Baby series whenever I have to deal with grids once more 😁).