in my experience, i've only seen the Romphalia spawn at Marunath when it had over 6 or 7k prosperity. I've also come across a unique Romphaia named Sledgehammer at marunath, but I'm unsure what causes that to spawn as I've seen it at both low and high prosperity lately.
But essentially, I think if marunath and the surrounding feifs are contested and constantly at war then there is a low, if no chance that the Romphalia will spawn. The Romphalia has been my go to horseback weapon since release, so i've always been on the lookout for it and have only ever found it at marunath with over ~6k prosperity.
In the current patch and beta, i think that it's going to be even harder to find it given how constant war is especially with the vlandians and battanians. I think this can just be said for any cities that are highly contested but I don't think the highest tier armors and weapons will spawn unless they meet a prosperity threshold.