Solution (for me)
I found out that CPU virtualisation can cause problems with the Marvell chip.
If you don't need it, deactivate it in BIOS.
I had the problem that my drives where reordered (sdX) between installation (Marvell worked) and regular boot (Marvell doesn't work).
After disabling VT-d and a new installation of OMV5, everything stays in place.
Sry i can't help you, but i can confirm your problem.
I have also an PCIe Adapter with an Marvell 88SE9230.
With OMV4 everything worked and also with a clean Debian 10 installation.
During installation of OMV5 (with the .iso) i see all discs and raids,
but after the first reboot, everything is gone.
I can only see the discs, connected to the internal Sata ports, but nothing connected to the Marvell PCIe card.
