As per title, kernel module dm_mod is not being loaded in the aformentioned kernel, therfore it will not deal with encrypted disks.
I don’t know if it has been replaced by something new, but if so it isn’t working.
lsmod | grep -i dm_mod dm_mod 155648 1 dm_crypt
4.20.8-1 kernel lsmod | grep -i dm_mod manjaro@manjaro ~>
You can of course modprobe it then it works, but it should be builtin.