Everything here is working and is expected behaviour for bbPress.
You are correct replies no longer have a title, the slug of a reply is now the reply ID.
You would typically only notice this working in the backend by not seeing a reply title in the ‘edit reply’, yet when you look at the list of replies in the backend just like bbPress you will see all the replies have a title, this title is inherited from the topic title.
The key thing here is that when viewing a single reply on the front end it has no context without it’s parent topic. We also encourage the use of the vast majority of bbPress via the front end and not the backend, the backend is really only meant to be used if you cannot do something via the front end, we try to make everything work via both interfaces but due to other issues further upstream in WordPress Core we can’t replicate all the functionality we implement.
As you stated via @JJJ until WordPress 3.7 replies had to have a title, this imposed some limitations on what we could and could not do with replies, further to this change in bbPress there will be even further enhancements coming to bbPress, maybe 2.6, if not 2.7 due to this new found freedom and functionality we have with our replies.
As I continue reading the previous page I knew this was coming 😉
Why do I want this?
1. permalink without reply-title, but only with reply-ID is not good for search engines.
Neither is duplicated content, previously if your topic URL was /topic/topic-title, then your replies would be /reply/topic-title-1, /reply/topic-title-2 etc, similar content but not the same content, now each reply has a unique URL.
2. if I open a reply-post in the admin area, there is no title “reply to: …. (topic title) …” (the title bar is empty) and I do not know to which post this reply refers. This is not good for handling the forum in practice.
This is one of those things I just mentioned above, about working via the frontend vs the backend and until the updates upstream flow from WordPress Core this field will remain empty until we can actually hide it completely. You should be editing replies via the frontend, moderators and users all use the frontend to perform all the tasks required. I’m only a moderator here on bbPress.org and can perform any task bbPress needs just fine, it is the preferred experience as just because you can do something in the backend does not always mean you should.
Apologies that I only saw this this morning, crazy week last week.
