Replies aren’t actually broken, I just… need to add myself to the follow list?! That’s quite counter-intuitive and (IIRC) not mentioned in the docs. But… It seems to be working now, which is nice (I still don’t know how webmentions and webfinger works, so can’t speak about this so far)
yarnd (what runs here at twtxt.net) actually does this automatically by default. I think it’s just an implementation detail to be honest. There’s nothing about this in the specs over at https://twtxt.dev