Did you ever get the matter resolved? I was experimenting with full NVMe flash S2D using Samsung PM1725b drives and found myself in the same situation. It seems like a driver issue, but Samsung doesn't provide any.
Not yet, I opened two cases with Microsoft to see if another support engineer might know a solution. The strange thing is that the disks locally without S2D clustering, works perfect. I did some testing with diskspd and tested the random read/write performance with PM1725 locally and the results are very good. I also tested the Intel P3700 SSD but that device is also not recognized as PLP device. Microsoft keeps trying to say it is something with your network but I don't think that is the issue. I will be redesigning my Storage Spaces Direct cluster with 24x Western Digital SS540 1.6TB SAS SSD. I will try the PM1725 as cache with that senario. I also bought some new network cards from chelsio to do iWARP instead of RoCE networking.
Here is the random read/write performance 4 Threads - 32 Queue - 4K
COmmand used for testing: DiskSpd.exe -r -c4G -d60 -w50 -t4 -o32 -b4K -L -Sh C:\tmp\testfile.dat
without S2D: Read (MB/s): 1046, Write (MB/s): 1019, Latency read: 476 μs, Latency write: 490 μs, IOPS read: 267.957, IOPS write: 260.948
with S2D and HDDs: Read (MB/s): 781,29, Write (MB/s): 89,6, Latency read: 640 μs, Latency write: 5572 μs, IOPS read: 200.009, IOPS write: 22.938
Here is the random read/write performance 1 Threads - 1 Queue - 4K
Command used for testing: DiskSpd.exe -r -c4G -d60 -w50 -t1 -o1 -b4K -L -Sh C:\tmp\testfile.dat
without S2D: Read (MB/s): 40,53, Write (MB/s): 149,11, Latency read: 26 μs, Latency write: 26 μs, IOPS read: 10.376, IOPS write: 38.172
with S2D and HDDs: Read (MB/s): 38,77, Write (MB/s): 1,89, Latency read: 100 μs, Latency write: 2061 μs, IOPS read: 9.924, IOPS write: 484
As soon as I redesigned the cluster I will reply here to send you the details.