However, the cruxt of the issue is that phrases are not guaranteed to be ok to contain HTML. It's very context dependent. Even if the first example didn't break out, HTML wouldn't work. The rule of thumb I would follow: unless a phrase contains HTML, assume it shouldn't contain HTML. This would actually be quite a few phrases then. Phrases that are literally a phrase or a few words should almost definitely not contain HTML.
A reasonable rule of thumb, except that in most instances the phrases are used many times, and for the majority of appearances HTML works fine. It will only be a single appearance, such as the usage for button title as above, where the HTML doesn't work. If XF is allowing HTML for phrases, then I believe it should use HTML stripping if it's going to use that phrase where HTML cannot be used.