I've also experienced the same issue. Unfortunately for me, my server boots from an array on one of my MegaRAID cards (there's two 9285CV-8e controllers in the system).
For now, my server (and its 200TB of storage) is stuck on 1909, unless someone knows a better solution?
I'm tempted to turn the current bare-metal install into a VM and run Hyper-V Server 2019 on the hardware (as it's based on an older build, this *should* run fine, I think?). An alternative to this might be to return the chassis to the original non-expander backplane so I can run SATA cables from four of the motherboard ports to it to allow me to use Windows Dynamic Disks (urgh software RAID!) to run the OS directly, so I can update the drivers, etc. with the MegaRAID cards disconnected, then put it back how it was after the upgrade. (that's an "if all else fails" option, as this is a "proper" server board with decent, modern IPMI, which I'd be wasting by switching to Hyper-V Server).
In case it's relevant, the server in question is running the following kit:
ASRock Rack X470D4U
Ryzen 5 3600 (with Dynatron A24 2U cooler)
32GB Corsair DDR4-3000 (2x16GB)
Supermicro SC826 chassis with 800W redundant PSUs
Windows 10 Pro
QLogic QLE8152 dual-port 10GbE CNA
(First) MegaRAID 9285CV-8e:
2x Samsung 850 Evo 500GB - configured in RAID-1 for base system OS
2x Samsung 850 Pro 512GB - configured in RAID-1 for the VM that runs my Plex install
24x 6TB SATA disks (mix of WD Red and Seagate Barracuda - mix of retail and shucked drives) - configured in RAID-60
(Second) MegaRAID 9285CV-8e:
12x 10TB WD Red (mix of retail and shucked drives)
The SSDs are in the front slots of the main chassis (using an SFF8087-SFF8088 adapter and a "loopback" cable), running via a Supermicro 6Gbps SAS expander I bought separately and fitted to this chassis.
The 36 other drives are housed in Xyratex HB1235E enclosures.
Incidentally, I've noticed a significantly higher failure rate on the 6TB WD Red drives than the Seagate Barracuda, despite both being SMR (which I was unaware of at the time of initial purchase). I've also had two of the WD Red 10TB disks fail within two years of purchase.
In case this helps @Nigel Johnstone, to successfully get the CacheVault (supercapacitor-flash backup) working properly after a firmware update, you also need to update the "gas gauge" firmware on the card(s). I've had to do this to several of my MegaRAID cards, including an M5016 (that I had previously written off as faulty - updating the gas gauge firmware fixed it!).