Interesting. I don’t know too much about Ruby/Rails, so sorry if this is obvious and I’m just not connecting the dots; but, if you go to the “New Comment” page directly (ie, not as a modal window), won’t the form be trying to post to a data-turbo-frame="modal" target?
If progressive enhancement is too important for you, I’d suggest to look at unpoly.js, it is better at PE than hotwire or htmx