Forgot to mention this quite some time ago, but just in case Halo 6 uses the same engine I’d like to go ahead and mention it now.
If you Join In Progress to a game and meet the qualifications for a Perfection (at least 15 kills and 0 deaths), then you won’t be awarded it. Found this out a few times when I was able to do so by more than was required. Just seems like a bug because a JIP has you spawn in on respawn points and not just utilizing the initial spawn points. I feel like that’s probably the root cause.