WD MyBook Live - firmware automatically reverted from OpenWRT to WD OEM?

A bit of history to set the scene...

Some time back I took the disk out of my WD MyBook Live, wiped it, and stuck OpenWRT on it. Can't remember the version, probably 17 or 18. After imaging, the disk had two partitions on it. I added a separate, third partition to store data, reasoning that, if I had to nuke the first two partitions as part of an ugrade/reinstall, the third partition would (hopefully) survive.

I don't remember if I left the disk as GPT, or if I changed it to MBR at the time.

Later on I upgraded to 19 without pulling the disk, by being cocky and mounting the .img as a loopback device and using dd to write the partition from the .img to the disk. I'm not entirely sure this was the best approach, but it seemed to work, and the MyBook Live kept doing duty as an impromptu NAS.

Most recently, I tried to be even more "clever" and attempted to upgrade from 19 to 21.02rcX (can't remember which release candidate) by flashing the sysupgrade.img.gz file through the OpenWRT GUI. And then promptly bricked the unit. It dropped off the network and wouldn't come back.

I hooked it up to a port-mirroring switch and started a packet capture to see if the thing might spew out any network traffic on boot, e.g. DHCP or some other broadcast, but without success. It wasn't announcing anything on the wire that I could observe. In addition, the front panel LED lit up white instead of blue.

Today I finally amassed enough round tuits to take the thing apart and hook the disk up via USB to query it directly.

And I got quite a surprise...

The disk doesn't have three partitions on it any more. It has four.

Disk /dev/sdb: 1.84 TiB, 2000398934016 bytes, 3907029168 sectors
Disk model: EURS-63S48Y0
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: 6B9F8103-93D0-4FA9-8576-A0C1F0370411

Device       Start        End    Sectors  Size Type
/dev/sdb1  1032192    5031935    3999744  1.9G Linux RAID
/dev/sdb2  5031936    9031679    3999744  1.9G Linux RAID
/dev/sdb3    30720    1032191    1001472  489M Microsoft basic data
/dev/sdb4  9031680 3907028991 3897997312  1.8T Microsoft basic data

Partition table entries are not in disk order.

The original WD factory installation has four partitions. In addition, the WD OEM firmware uses software RAID, hence the question posed in the subject line of this post. If I completely knackered the system with my cavalier approach to upgrading OpenWRT, would the WD board attempt to restore the original factory software (or, at least, the original partition table) in an optimistic attempt at recovery?

Ultimately, I'd kind of like to find and preserve the data in that third partition I originally created, but I appreciate that might not be possible. If so, that'll be irritating (takes a while to populate the disk with all the music I want it to hold), but not the end of the world.