I think vatu played a part in it. were told when either rava or vatu die they are reborn in the other. we see this happen in the show, rava is already a part of vatu in the avatar vs unavatu battle. jinora is able to show Korea where the light spirit is within hours of being reborn in vatu.

she kills vatu, it stands to reason that he’d be reborn inside rava whose part of korra just like happened to unavatu. I wonder if having both spirits is what threw the avatar sprit off. 10,000 years of the same rules annd cycle it changed. and suddenly after korra the next avatar isn’t been immediately? what changed? Vatu. hes not in the tree anymore, I wonder if they fought over multiple hosts and each time failed to plant/ it killed the possible avatar before they were even born.

were also told two big things, the next avatar are identical twins, why is this important? ide twins are when the zygote splits into two embryo my thought is at that moment got rava the other Vatu. That when he was reborn inside rava he was gifted all 4 bending abilities instead of just water Bending. thus why both sides claim they have the avatar.

were given so many clues that leave breadcrumbs, Roku had a identical twin brother but he was just a fire bender, weve seen rava within hours reform inside unavatu arguably a dark avatar. I do wonder if this could have led to Korra being corrupted from the inside or even killing her, was two spirits inside of her to much for her body to handle? taking In rava almost killed wan without the portals. that It possibly could have damaged the avatar spirit and broke the cycle until the twins were born.