On closer inspection it appears that ESXi 6.5U1 is NOT reading partitions properly. Below, I've dumped the partitions via UI and esxcli. The UI shows the partition as <blank> AND the diagram as being correct - bizarre. There's something SERIOUSLY WRONG with this update. esxcli shows the same partition tables on both builds, and the volume continues to work correctly on the '538 build.
Here's the output on both builds of ESXi 6.5 ...
ON BUILD 5969303 HOST
Web UI
FreeNAS iSCSI Disk (naa.6589cfc000000bcff79a34187f94af3a)
Type: Disk
Model: iSCSI Disk
Path: /vmfs/devices/disks/naa.6589cfc000000bcff79a34187f94af3a
Capacity: 400 GB
Partition Format: gpt
UUID: 010003000030303063323961373930363631360000695343534920
Partitions
<blank>
Partition diagram
1. VMFS (400 GB)
esxcli storage core device partition list
naa.6589cfc000000bcff79a34187f94af3a 0 0 838860800 0 429496729600
naa.6589cfc000000bcff79a34187f94af3a 1 128 838860767 fb 429496647168
ON BUILD 5310538 HOST
Web UI
Type: Disk
Model: iSCSI Disk
Path: /vmfs/devices/disks/naa.6589cfc000000bcff79a34187f94af3a
Capacity: 400 GB
Partition Format: gpt
UUID: 010003000030303063323961373930363631360000695343534920
Partitions
1: VMFS
Type VMFS
Start block 0
End block 409599
Block size 1 MB
Size 400 GB (409599 blocks)
Partition diagram
1. VMFS (400 GB)
esxcli storage core device partition list
naa.6589cfc000000bcff79a34187f94af3a 0 0 838860800 0 429496729600
naa.6589cfc000000bcff79a34187f94af3a 1 128 838860767 fb 429496647168
