mdadm fails to assemble IMSM - Cannot assemble mbr metadata on /dev/md126

/* General Description ******************************************************/

I'm having a problem using mdadm in a "fakeraid IMSM" environment. It seems like the container Volume0_0 is found and assembled but the partitions (x3) are never put together. The setup works properly when manually using a mix of dmraid for IMSM and mdadm for my other RAID1 regular linux software raid devices.

/* System *******************************************************************/

Ubuntu 16.10
mdadm 3.4-4

/* Debug Information ********************************************************/

Please consider the following debug trace during initramfs boot up:

/dev/sdb is identified as a member of /dev/md/imsm0, slot -1.
/dev/sda is identified as a member of /dev/md/imsm0, slot -1.
added /dev/sda to /dev/md/imsm0 as -1
added /dev/sdb to /dev/md/imsm0 as -1
Container /dev/md/imsm0 has been assembled with 2 drives
looking for devices for further assembly
[...]
looking in container /dev/md/imsm0
found match on member /md127/0 in /dev/md/imsm0
Started /dev/md/Volume0_0 with 2 devices
looking for devices for further assembly
[...]
looking in container /dev/md/imsm0
member /md127/0 in /dev/md/imsm0 is already assembled
looking for devices for further assembly
[...]
looking for devices for further assembly
Cannot assemble mbr metadata on /dev/md126
[...]

Addtionnal info : GParted is complaining that "Not all of the space available to /dev/sda appears to be used, you can fix the GPT to use all of the space (an extra 50016680 blocks) or continue with the current setting?", and I also click ignore. Same applies for /dev/sdb.

/* Disks information ********************************************************/

IMSM RAID1 on /dev/sda and /dev/sdb SSDs (GPT with protective MBR)

DMRAID output (working, for reference only)

isw_hggicbcjd_Volume0
- isw_hggicbcjd_Volume0p1 - mounted on /boot/efi - vfat
- isw_hggicbcjd_Volume0p2 - mounted on /boot - ext4
- isw_hggicbcjd_Volume0p3 - mounted on / - LVM on luks

/* Workaround ***************************************************************/

To boot my system, I go through these steps to fallback on dmraid :
kernel option break=mount

from initramfs prompt
- mdadm --stop /dev/md126
- mdadm --stop /dev/md127
- dmraid -ay
- cryptsetup luksOpen /dev/mappper/isw_hggicbcjd_Volume0p3 isw_hggicbcjd_Volume0p3_crypt
- CTRL-D

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INITIALLY REPORTED HERE
https://ubuntuforums.org/showthread.php?t=2353942