The metadata really looks all ok, but mdadm fails to assemble it
correctly. It does look like a bug to me. The problem seems to be
related to the global spare somehow. I have no idea why.

The best thing to proceed would be to run mdadm --dump on
/dev/sd[bcdef], put the output in a tgz and make it available to us
somewhere. So we could actually debut the assembly.

Regards,
Martin