Is this behavior documented anywhere on news.ycombinator.com?
https://news.ycombinator.com/newsguidelines.html
and
https://news.ycombinator.com/newsfaq.html
and a handful of posts by dang, sama, pg, etc. over the course of the years. most of the rest is what long-time users have just figured out through observation. There's a Git repo[1] out there that aggregates a lot of that stuff, but keep in mind that it's technically unofficial. That said, I think most of what's there is widely considered to be correct.
I can't find any reference to something like "combine the scores of new submissions of the same URL to the first submission's score" though. I guess that's either new information or incorrect.
I think that falls into the "noticed through observation" bucket. I'm relatively sure that it is correct, as I've noticed that behavior myself. But, I have no official standing here and I could be totally wrong. But that sure seems to be what happens in my experience.