Comment # 17 on bug 1202589 from Walter Haidinger

Well, for a start I sent SIGRTMIN+22 (loglevel=debug) and SIGRTMIN+27
(log=console) to systemd right before reboot. Signals are described in
systemd(1).  Will look into for howto enable specific udev debugging.

Also reattached md2 as a spare for md4. A sync error is shown now.
What's odd is that the system now did return to initrd...
Captured serial console logs:

Couldn't finalize remaining  DM devices, MD devices, trying again.
Stopping MD devices.
sd-device-enumerator: Scan all dirs
sd-device-enumerator: Scanning /sys/bus
sd-device-enumerator: Scanning /sys/class
Stopping MD /dev/md2 (9:2).
Failed to sync MD block device /dev/md2, ignoring: Input/output error
Stopping MD /dev/md1 (9:1).
Could not stop MD /dev/md1: Device or resource busy
Not all MD devices stopped, 1 left.
Detaching DM devices.
sd-device-enumerator: Scan all dirs
sd-device-enumerator: Scanning /sys/bus
sd-device-enumerator: Scanning /sys/class
Not all DM devices detached, 1 left.
Couldn't finalize remaining  DM devices, MD devices, trying again.
Stopping MD devices.
sd-device-enumerator: Scan all dirs
sd-device-enumerator: Scanning /sys/bus
sd-device-enumerator: Scanning /sys/class
Stopping MD /dev/md2 (9:2).
Failed to sync MD block device /dev/md2, ignoring: Input/output error
Stopping MD /dev/md1 (9:1).
Could not stop MD /dev/md1: Device or resource busy
Not all MD devices stopped, 1 left.
Detaching DM devices.
sd-device-enumerator: Scan all dirs
sd-device-enumerator: Scanning /sys/bus
sd-device-enumerator: Scanning /sys/class
Not all DM devices detached, 1 left.
Couldn't finalize remaining  DM devices, MD devices, trying again.
Stopping MD devices.
sd-device-enumerator: Scan all dirs
sd-device-enumerator: Scanning /sys/bus
sd-device-enumerator: Scanning /sys/Successfully changed into root pivot.
Returning to initrd...
dracut: Taking over mdmon processes.
dracut Warning: Killing all remaining processes
EXT4-fs (dm-1): unmounting filesystem.
dracut Warning: Unmounted /oldroot.
dracut: Disassembling device-mapper devices
dracut: Waiting for mdraid devices to be clean.
dracut: Disassembling mdraid devices.
md1: detected capacity change from 440139776 to 0
md: md1 stopped.
mdadm: stopped /dev/md1
Rebooting.