Through a recent patch, os-prober seems to have been disabled by default. See here for details: https://gitlab.manjaro.org/packages/core/grub/-/commit/991784fbecb814a14651e1ccbd0dd6b08de2a129
I had the same issue and could resolve it by manually adding the line GRUB_DISABLE_OS_PROBER=false
to /etc/default/grub.