Why does this happen often when I quote someone?

Often (not always, but more often than it doesn’t) when I quote someone and post, the quote disappears and I get a notification that has a pencil icon (that means “edited” when hovering over it) and says “system (topic name)”.

Why do my replies often automatically get edited like this?

I don’t know. I get annoyed when this happened too.
You can then edit the post, re-add the quote, and then it will stick … but still, it’s annoying.

My experience is that this only happens when you quote a complete post, never when you quote a part of a post. So that’s what I now use as a workaround.