I had the same problem — after updating to WordPress 5.5, comment-reply.js seemed to stop working because replying became “manual” instead of dynamic.
Followed @themerex’s comment by adding id="reply-title" to my theme’s markup (I didn’t have it because I customized this title).
comment-reply.js is looking for that ID in the markup — and if it doesn’t find it, it will stop working.
