EXT4 partition became empty after reboot - why? possible to fix?

TLDR: I had a 1TB RAID1 partition containing 800GB of files before rebooting, now after rebooting, the partition is empty.

Hardware: dual SATA 1TB drives in RAID1 (ASUS PIKE card, which is a LSI card). O/S is on a separate 320GB SATA drive, not part of the RAID array.

O/S: Centos 7.6.1810 (x64)

Scenario: I cleanly powered off the system (poweroff). When powering back on, the system boots from the main OS partition (not on the RAID array), and mounts the RAID partition on /mnt/sdb1. But the /mnt/sdb1 directory contains NOTHING except the "lost+found" directory and a .Trash-1000.

df -Th reports: /dev/sdb1 ext4 917G 77M 870G 1% /mnt/sdb1

RAID firmware reports the array is functioning optimally (no failures, no out-of-sync errors)

fsck.ext4 reports: /dev/sdb1: clean, 16/61038592 files, 3884291/244140288 blocks

I attempted to restore various superblock backups but this yields no difference in files.

mke2fs -n /dev/sdb1 reports:

Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
Stride=0 blocks, Stripe width=0 blocks
61038592 inodes, 244140288 blocks
12207014 blocks (5.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=4294967296
7451 block groups
32768 blocks per group, 32768 fragments per group
8192 inodes per group
Superblock backups stored on blocks:
        32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208,
        4096000, 7962624, 11239424, 20480000, 23887872, 71663616, 78675968,
        102400000, 214990848

When restoring any of these, it then reports that the partition hasn't been cleanly unmounted and proceeds to run fsck, which reports some errors, which I "fix" when prompted.

After reboot and remount - still shows the lost+found directory and nothing else.

I tried the testdisk utility and it reports the ext4 partition but shows the following when attempting to list files:

   P ext4                     0   0  1 121576  93 53 1953124352 [RAID1]
Directory /

No file found, filesystem may be damaged.

The data is backed up, so it's not lost, but I really want to know why this happened, as I was under the impression ext4 was more resilient than this.

Thanks for any ideas or suggestions.