PASETO and TLS 1.3 were also written by humans. TLS libraries (which are several orders of magnitude more complicated than JWT libraries) are also written by humans.
If you passionately care about security and misuse-resistance you CAN write a spec that will lead to fewer implementation issues.
You must be young if you're pointing to TLS libraries as an example of doing it right and not getting into trouble with insecure implementations and downgrade attacks. https://wiki.freebsd.org/LibreSSL
I wish I was young. Did I explicitly said TLS __1.3__ or did I not?
A lot of effort was put into making TLS 1.3 a stronger, less agile and more misuse-resistant standard than its previous iterations. And that effort worked.