Any new updates on this?
Booting into Core showed the same state of pool/disks as Scale. The disks are done, labels cannot be created in any way, I’ve tried ZFS with TrueNAS, ext4 with OpenMediaVault, none of them can create labels or working partitions. I’ve fully zeroed the disks already, didn’t help. Only thing that works is NTFS partitions under Windows, those work fine.
If you can partition and create NTFS filesystems on both disks, then it means the disks were fine. Something else had happened.
I really doubt that both disks simultaneously “died” coincidentally just after a TrueNAS upgrade, but then work again when formatting them in Windows.
I think “something” happened after the upgrade which ruined the label/partition info, making the pool unimportable.
Since the disks have been partitioned and formatted again, and you recovered your files with UFS Explorer, I guess there’s nothing left to test. The beginning of the disks have essentially been overwritten and there’s no dd’d image of their early LBAs to play with.
Glad you got your data back. 
prevention is better than the cure. hopefully now you realize the importance of maintaining a backup for situations like this.