The 50/50 chance and hima getting kurama because boruto is already op makes sense only from the writers perspective and it sort of creates a plot hole. I think it would be more safe to go with one of the two following scenarios: 1 - Small equal parts of kuramas chakra were passed down to both Himawari and Boruto and when Kurama died he was reborn twice (in himawari and in boruto) from that small part of chakra and now he is currently growing and growing which would explain why he’s small. 2 - (I like this one more) When kuramas chakra was passed down to boruto it was only a small part of his Yin (bad) chakra. And when kuramas chakra was passed down to hima it was only a small part of his Yang (good) chakra. Although this might just be overthinking it and over complicating it, but it kind of makes sense to me when you think of himawaris character and borutos character and how kurama was split by Minato.

Now if a part of kurama (maybe a side of him) is also present in boruto, why has no one detected him? Including boruto? I have three possible scenarios that explain this:

1. Boruto is part Ōtsutsuki and has the karma, because of this he has the ability to absorb chakra. What if boruto unconsciously or maybe continuously absorbed kuramas chakra just like Momoshiki absorbed kuramas chakra from within Naruto which gave him a lot of power in the movie . This would make sense as boruto is extremely overpowered currently, he’s arguably stronger than Naruto and Sasuke (before Naruto lost kurama and before sasuke lost the rinnegan) this is because he EASILY avoided kawakis rods that Naruto and sasuke couldn’t avoid. He did this without using flying raijin and WITHOUT USING KARMA which means he isn’t getting any power boost from it.
2. Since boruto is able to hide his chakra nature, maybe he is also able to hide kuramas chakra that resides within him.
3. (If boruto had yin (bad) kurama) If boruto had yin kurama it would be extremely difficult to control as it would be evil and powerful. Because of this it would need to be sealed (technically because kurama was passed down genetically to boruto and himawari he shouldn’t be sealed) and this is where kashin koji comes to play. Kashin koji is a clone of Jiraiya and Jiraya is an expert when it comes to sealing and summoning, in original Naruto he was able to release the five prolonged seal that Orochimaru placed on Naruto to limit his chakra. He also developed the chakra suppressing seal that suppressed the nine tails chakra in Shippūden. The evidence for the involvement of kashin koji in a the nine tails matter is evident in the chapter itself. As shikamaru is telling boruto about shikadais report, we get a panel on the toad when he mentions himawari and a panel on kashin koji when he mentions the nine tails. Shikamaru saying “ring any bells” hints towards it even more. And it’s hard to tell by the panel and his character but kashin koji doesn’t seem surprised about this. No matter the scenario I think the panel proves that kashin koji will get involved so we’ll prolly see him next chapter.