I think that may be (possibly?) due to the <code> tags surrounding the UA string triggering a false alarm and the user dismissing the warning.
I can blanket whitelist text within <code> tags if needed, but actually I wonder if that’s a good idea — it’s not really the correct way to format code on Discourse. For example, <code><span></span></code> renders as that exact HTML, when the intention was almost certainly to render <code><span></span></code> instead.
[Edit: Opened a PR against this on the fCC repo]