Thank you for reporting back with useful information as there are too few MCH owner who does. As to “WD had no idea”, I won’t be too sure as they did an high-wire act with their upgrading an OS (OS4) from Android to Debian and kept the old volume and data in partition 24 of the HDD - what that did was created a whole slew of permission problems with folders and files created by old apps such as Plex and SMB clients (Windows and macOS) that may no longer be written again by the new admin of Debian OS4. So of course WDC knew, it is just that they can’t offer much help other than asking the user to reset to factory as some files without the proper write permission would appeared to have written to a folder in a volume but will be soon deleted.
But you did exactly the right solution, backing up the MCH before you lose anymore data and reset and repopulate your disk from backup data.
As to this situation being unique to MCH or WD, it is not - it is often seen in upgrades of other NAS where the old volume became incompatible with the new OS and their solution is the same, this one from Synology:
