I'm posting this here because honestly I'm not sure where to post it exactly. The way this happens I think it's related to cross-posted threads but I'm not very sure.
Steps to reproduce the error:
1-) In a cross-posted article in the front page a user replies to one of the articles.
2-) Because the posted message is not related to the article in question (Unrelated question and the sort) a moderator uses the forum moderation tools to split the specific message and convert it into a new post in the forum.
3-) The moderator then goes to the original front page article and deletes the original message posted by that user. Everything is ok until this point.
4-) If you then go to your comments trash and delete this message from the trash (Or if you use any plugin which cleans junk) the new posted that was created in the forum by that moderator (including all following replies, pictures and all data) will be deleted from the forum.
That's awful and has caused me to lose several important posts in the forum, I was fighting with my moderators thinking they were deleting posts they didn't like etc lol.
Ideally I think if a moderator creates a new thread in the forum that way it should then automatically delete that message from the front page article as well.
I have tested this on my staging site as well as my live site and I have the exact same results.
Let me know if you need login details to debug this.