Hi,
There is a character limit in that field, which is actually part of the Summary Field, so yes, it does happen that an author can't add a tag that they forgot when posting the story. In that case, we are fine with an author's note at the beginning of the first chapter that adds any additional warnings and tags. This way, readers see it first thing, and can choose not to read if this is a topic that would upset them.
Just add the tags at the beginning of chapter one, and we'll accept that!