Byleth, Crimson Flower and Buddhism.
Before i begin, i want to make it clear, i'm not looking for a discourse.
So, i have seen some theories regarding Byleth regaining back their heart beats, thus becoming a human in the CF ending, and it's relation to Buddhism. It has to do with Byleth's "enlightened one" (or nirvana in jp) class. The class signifies Byleth as a reference to Buddha, as a Buddha is one that has reached enlightenment/nirvana, and their role is to teach/guide people to reach the enlightment too. I have seen them not only here, but in several other forums and social media.
The most popular theory about why they became human then, is that, it is a punishment for Byleth, for failing to be the teacher Fodlan needs. Fodlan needed someone to guide them into enlightenment, but Byleth failed, because they helped killed a god (Rhea), so they were reduced to a mere human. I have no idea who put on this idea first, but this is a wrong take on Buddhism. It is in fact, quite insulting for Buddhists.
It is insulting for Buddhists, because those people said, according to Buddhism teaching, a Buddha can turn back into a human if a Buddha does grave sins, (as proven by Byleth turning into a human), thus basically saying Buddhism teaches a Buddha is still capable of sins and earthly desires. NO. A Buddha can't turn back into a human, as to become a Buddha, one has let go all earthly desires and causes. A Buddha is incapable of sins. There is simply no way. Nowhere in Buddhism teachings teaches a Buddha can turn back into a human.
So no, Byleth didn't turn into a human because they were punished.
Please keep in mind, that Buddhism is NOT the same with confucianism or whatever religions that have Buddhism mixed with other believes. I have seen someone used Monkey King (Journey to the West) reference to explain Buddhism in CF. Monkey king is not Buddhism! It's a mixed of a bit of Buddhism and local chinese folk believes and many others, plus it's a fictional novel.
So then, why did Byleth become a human?
Now, Buddhism itself has many branches. I myself learned Theravada teaching, thus i didn't understand either at first why Byleth became a human in CF in regard of Buddhism reference. It wasn't until i spoke with a friend, i came to learn that Japanese Buddhism is actually of Mahayana teaching. And that changed everything.
You see, in Buddhism, there is one thing called Bodhisattva. In Theravada, that simply means a person on the path becoming a Buddha. But, in Mahayana, that has entirely different meaning. In Mahayana teaching, a Bodhisattva is a person who is already enlightened, thus eligible to be a Buddha, but postpone it due to their compassion and desire to help others, so they stay in the cycle of death and rebirth (samsara). They can take many forms in samsara, as a human, animal, anything that is beneficial in purpose to help others, all the while STILL retaining their enlightenment. Their status doesn't change.
They just decide not to become a Buddha yet, because they feel they will be more helpful in other forms. As once you become a Buddha, there is no way you can be back into a human or any other creature. (This part where "once you become a Buddha, you can never go back and become a human or any other creature" teaching is the same in Theravada and Mahayana)
Byleth in CF is a reference to Mahayana's Bodhisattva. Byleth was already enlightened, but in the end they didn't become a Buddha (or god, as fe3h fanbase refer it, but for a side note, in Buddhism, a Buddha is not a god) yet, instead, they became a human, because it is the form which will be most beneficial to help others in CF. It is in lieu with how Edelgard in CF stated as long as humans stand together, there is no need of gods.
Byleth still retain their enlightenment in CF, they still act as a teacher to guide others to reach enlightenment, they just do it in human form. They did not fail their purpose to be the teacher Fodlan needed and loose their enlightenment. They are still the enlightened one in human form - just as a Bodhisattva does.
Now, whatever take you prefer in explaining why Byleth became a human, be it simply to say they regained their humanity, or it is a reference to Buddhism's Bodhisattva, is up to you, but, please, do not say it is a Buddhism teaching as a punishment because Byleth had failed, so they got reduced from a Buddha to a human. It is quite insulting.