@baris thanks I'll pull it tomorrow from git and fix the categories as you've said. It's not a big problem, more an oddity that occurred - thank you. And I really should raise bugs like this on GitHub!
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).
The author also could create a new post stating that some changes have been made if it's worth an update in his opinion. I don't think any other concept is needed since editing posts is more wiki-like than forum-like. If your forum relies on something like this you might consider using a wiki platform instead or additionally.