I think this is plugin territory. In my forum I have several threads where only admins can post updates for users. Not really a discussion topic, more of a change log or mini blog. If posts were to start merging the updates would get tangled.
Just one use case, but I'd prefer the core code not to automatically join double posts....or at least make it an option.
You know that if you edit a file of NodeBB itself it may refuse to update by git pull because a merge would be needed?
So it may be that you're working with an old version of NodeBB o_O
If you are able to delete a post you have the permission to also restore it. Because of this the post doesn't get removed but only gets less opacity (within the official themes).
I just don't know in which case the post could be removed from DOM after reload, may be an inofficial theme or an old version of NodeBB (maybe because of the reason above).