RAID not mounted after reboot

After a system update, I restart the router manually (using Luci)).

When the system restarts, the RAID drive hasn’t been mounted:

root@turris:~# df -T
Filesystem           Type       1K-blocks      Used Available Use% Mounted on
/dev/mmcblk0p1       btrfs        7633920   2423704   5255276  32% /
tmpfs                tmpfs         515472      3188    512284   1% /tmp
tmpfs                tmpfs            512         0       512   0% /dev
/dev/sdc2            hfsplus    488216620    437708 487778912   0% /mnt/samsung
/dev/sdc1            vfat          201633         1    201633   0% /tmp/run/mountd/sdc1

If I restart the router a second time, the RAID volume will be mounted:

root@turris:~# df -T
Filesystem           Type       1K-blocks      Used Available Use% Mounted on
/dev/mmcblk0p1       btrfs        7633920   2423996   5254984  32% /
tmpfs                tmpfs         515472      2476    512996   0% /tmp
tmpfs                tmpfs            512         0       512   0% /dev
/dev/sdc2            hfsplus    488216620    437708 487778912   0% /mnt/samsung
/dev/sdc1            vfat          201633         1    201633   0% /tmp/run/mountd/sdc1
/dev/md0             btrfs      1953383360 672806764 1279536948  34% /mnt/raid

What’s happening and how do I fix it?