sel4 is amazing, but it is not bug free:
https://github.com/seL4/seL4/issues/36
It might be close... for miTLS I don't have access to the issues, but let's assume it's bug free now for sake of argument; it hasn't always been bug free, that is in earlier unproven releases.
"Program testing can be used to show the presence of bugs, but never to show their absence!" - Edsger Dijkstra