Well, that’s the behavior of the game nowadays. Nothing is really explained in the game itself.
But they said and announced it:
You cannot quote an identical post… the forum removes it and count it as if you had edited your post. But if you change something (partial quote by example) it will keep it.
This is an annoying thing on this forum, like the new feature that close automatically a thread after 7 days… 2 weeks ago I had posted in the bug forum, but I would want to add something, but I cannot except if I create a new thread… I will surely do that.