Ubuntu – RAID array doesn’t reassemble after reboot

RAID array doesn't assemble after reboot.

I have one SSD from which the system is booted, and three HDD that are part of the array. The system is Ubuntu 16.04.

The steps that I've followed are based mostly on this guide:

https://www.digitalocean.com/community/tutorials/how-to-create-raid-arrays-with-mdadm-on-ubuntu-16-04#creating-a-raid-5-array

1. Verifying if I'm good to go.

   lsblk -o NAME,SIZE,FSTYPE,TYPE,MOUNTPOINT
   

The output shows sda, sdb, and sdc devices besides the SSD partitions. I've verified if these in fact represent HDDs by looking at output of this:

hwinfo --disk

Everything matches.

2. Assembling the array.

   sudo mdadm --create --verbose /dev/md0 --level=5 --raid-devices=3 /dev/sda /dev/sdb /dev/sdc
   

I verify if it looks OK by entering: cat /proc/mdstat

The output looks something like this:

Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10]
md0 : active raid5 sdc[3] sdb[1] sda[0]
      7813774336 blocks super 1.2 level 5, 512k chunk, algorithm 2 [3/2] [UU_]
      [=======>.............]  recovery = 37.1% (1449842680/3906887168) finish=273.8min speed=149549K/sec
      bitmap: 0/30 pages [0KB], 65536KB chunk

unused devices: <none>

I wait till the process ends.

Personalities : [raid1] [linear] [multipath] [raid0] [raid6] [raid5] [raid4] [raid10] 
md0 : active raid5 sdc[3] sdb[1] sda[0]
      209584128 blocks super 1.2 level 5, 512k chunk, algorithm 2 [3/3] [UUU]

unused devices: <none>

3. Creating and mounting the filesystem.

   sudo mkfs.ext4 -F /dev/md0
   
   sudo mkdir -p /mnt/md0
   
   sudo mount /dev/md0 /mnt/md0
   
   df -h -x devtmpfs -x tmpfs
   

I put some data in, and the output looks like this:

Filesystem      Size  Used Avail Use% Mounted on
/dev/nvme0n1p2  406G  191G  196G  50% /
/dev/nvme0n1p1  511M  3.6M  508M   1% /boot/efi
/dev/md0        7.3T  904G  6.0T  13% /mnt/md0

4. Saving the array layout.

   sudo mdadm --detail --scan | sudo tee -a /etc/mdadm/mdadm.conf
   
   sudo update-initramfs -u
   
   echo '/dev/md0 /mnt/md0 ext4 defaults,nofail,discard 0 0' | sudo tee -a /etc/fstab
   

5. Rebooting and verifying if everything works correctly.

After reboot I try:
cat /proc/mdstat
It isn't showing any active raid devices.

ls /mnt/md0 

is empty.

The following command isn't printing anything and doesn't work either:

mdadm --assemble --scan -v

Only the following restores the array with data on it:

sudo mdadm --create --verbose /dev/md0 --level=5 --raid-devices=3 /dev/sda /dev/sdb /dev/sdc

What should be done differently?

Additional, probably useful info:

sudo dpkg-reconfigure mdadm

The output shows:

update-initramfs: deferring update (trigger activated)
Generating grub configuration file ...
Warning: Setting GRUB_TIMEOUT to a non-zero value when GRUB_HIDDEN_TIMEOUT is set is no longer supported.
Found linux image: /boot/vmlinuz-4.4.0-51-generic
Found initrd image: /boot/initrd.img-4.4.0-51-generic
Found linux image: /boot/vmlinuz-4.4.0-31-generic
Found initrd image: /boot/initrd.img-4.4.0-31-generic
Adding boot menu entry for EFI firmware configuration
done
update-rc.d: warning: start and stop actions are no longer supported; falling back to defaults
Processing triggers for initramfs-tools (0.122ubuntu8.5) ...
update-initramfs: Generating /boot/initrd.img-4.4.0-51-generic

The intriguing part for me is "start and stop actions are no longer supported; falling back to defaults"

Also the output of /usr/share/mdadm/mkconf doesn't print any arrays at the end.

# mdadm.conf
#
# Please refer to mdadm.conf(5) for information about this file.
#

# by default (built-in), scan all partitions (/proc/partitions) and all
# containers for MD superblocks. alternatively, specify devices to scan, using
# wildcards if desired.
#DEVICE partitions containers

# auto-create devices with Debian standard permissions
CREATE owner=root group=disk mode=0660 auto=yes

# automatically tag new arrays as belonging to the local system
HOMEHOST <system>

# instruct the monitoring daemon where to send mail alerts
MAILADDR craftinity@craftinity.com

# definitions of existing MD arrays

whereas the output of cat /etc/mdadm/mdadm.conf does.

# mdadm.conf
#
# Please refer to mdadm.conf(5) for information about this file.
#

# by default (built-in), scan all partitions (/proc/partitions) and all
# containers for MD superblocks. alternatively, specify devices to scan, using
# wildcards if desired.
#DEVICE partitions containers

# DEVICE /dev/sda /dev/sdb /dev/sdc

# auto-create devices with Debian standard permissions
CREATE owner=root group=disk mode=0660 auto=yes

# automatically tag new arrays as belonging to the local system
HOMEHOST <system>

# instruct the monitoring daemon where to send mail alerts
MAILADDR craftinity@craftinity.com

# definitions of existing MD arrays

# This file was auto-generated on Sun, 04 Dec 2016 18:56:42 +0100
# by mkconf $Id$

ARRAY /dev/md0 metadata=1.2 spares=1 name=hinton:0 UUID=616991f1:dc03795b:8d09b1d4:8393060a

What's the solution? I've browsed through half the internet and no one seems to have the same problem.

I've also added the exact same question on serverfault a couple of days ago (no answer). I apologize if I violated stack exchange's community rules by doing that.