mdmon is not being compiled and installed and there does not seem to be an option to enable it in through config. Turns out that in some cases we can't properly assemble raid1 arrays because of mdmon missing. A simple quickfix would be to add 'mdmon' (line 14) and 'install-mdmon' (line 17) in https://git.buildroot.net/buildroot/tree/package/mdadm/mdadm.mk
Fixed by https://git.buildroot.org/buildroot/commit/?id=45498bbc62d8dfbaa337854a93c379fe3aed1cd8. Thanks!