Tested the 4.17 kernel, and this is the issue I now see:

[...]
[   17.340040] raid6: sse2x1   xor()  2828 MB/s
[   17.408036] raid6: sse2x2   gen()  7445 MB/s
[   17.472037] raid6: sse2x2   xor()  5425 MB/s
[   17.540032] raid6: sse2x4   gen() 10259 MB/s
[   17.604037] raid6: sse2x4   xor()  6286 MB/s
[   17.668034] raid6: avx2x1   gen() 14089 MB/s
[   17.732034] raid6: avx2x1   xor()  9346 MB/s
[   17.796035] raid6: avx2x2   gen() 16758 MB/s
[   17.860033] raid6: avx2x2   xor()  9779 MB/s
[   17.924034] raid6: avx2x4   gen() 19079 MB/s
[   17.988036] raid6: avx2x4   xor() 11383 MB/s
[   18.007313] raid6: using algorithm avx2x4 gen() 19079 MB/s
[   18.031519] raid6: .... xor() 11383 MB/s, rmw enabled
[   18.054148] raid6: using avx2x2 recovery algorithm
[   18.076927] xor: automatically using best checksumming function   avx
[   18.109754] async_tx: api initialized (async)
done.
Begin: Running /scripts/init-premount ... done.
Begin: Mounting root file system ... Begin: Running /scripts/local-top ... done.
Begin: Running /scripts/local-premount ... [   18.199553] Btrfs loaded, crc32c=crc32c-intel
Scanning for Btrfs filesystems
done.
Begin: Waiting for root file system ... Begin: Running /scripts/local-block ... mdadm: No devices listed in conf file were found.
done.
mdadm: No devices listed in conf file were found.
mdadm: No devices listed in conf file were found.
mdadm: No devices listed in conf file were found.
mdadm: No devices listed in conf file were found.
mdadm: No devices listed in conf file were found.
mdadm: No devices listed in conf file were found.
mdadm: No devices listed in conf file were found.
mdadm: No devices listed in conf file were found.
mdadm: No devices listed in conf file were found.
mdadm: No devices listed in conf file were found.
mdadm: No devices listed in conf file were found.
mdadm: No devices listed in conf file were found.
mdadm: No devices listed in conf file were found.
mdadm: No devices listed in conf file were found.
mdadm: No devices listed in conf file were found.
mdadm: No devices listed in conf file were found.
mdadm: No devices listed in conf file were found.
mdadm: No devices listed in conf file were found.
mdadm: No devices listed in conf file were found.
mdadm: error opening /dev/md?*: No such file or directory
mdadm: No devices listed in conf file were found.
mdadm: No devices listed in conf file were found.
mdadm: No devices listed in conf file were found.
mdadm: No devices listed in conf file were found.
mdadm: No devices listed in conf file were found.
mdadm: No devices listed in conf file were found.
mdadm: No devices listed in conf file were found.
mdadm: No devices listed in conf file were found.
mdadm: No devices listed in conf file were found.
mdadm: No devices listed in conf file were found.
mdadm: No devices listed in conf file were found.
done.
Gave up waiting for root file system device.  Common problems:
 - Boot args (cat /proc/cmdline)
   - Check rootdelay= (did the system wait long enough?)
 - Missing modules (cat /proc/modules; ls /dev)
ALERT!  UUID=bd0dbca3-60b8-474e-ac82-96f1b9ff9cc2 does not exist.  Dropping to a shell!


BusyBox v1.27.2 (Ubuntu 1:1.27.2-2ubuntu3) built-in shell (ash)
Enter 'help' for a list of built-in commands.