I’m not the person you’re replying to but I’ve also been wondering the same. The best I can come up with is that “flue” is an actual word (like a chimney flue)

I’m not the person you’re replying to but I’ve also been wondering the same. The best I can come up with is that “flue” is an actual word (like a chimney flue) and autocorrect doesn’t alert people to the error. Kind of like there, their, and they’re. In writing this comment though I’ve learned that my iPhone autocorrects flue to glue. Even when following the word chimney. So perhaps my pet theory is totally wrong.