Well, it appears that the issue is the Rivo's design. It uses a Marvell PCIe SATA controller to provide 2 ports on the card. It has a second chipset, a JMB583 which is a SATA controller/port-multiplier to allow the remaining 6 ports to operate and share bandwidth. I can't find out anything about this chip as it isn't listed on the Jmicron website; their numbering stops at JMB575. Either the website isn't up to date, or this is a chip made for Rivo (and whoever else?). The Rivo driver, for Win10 and lower, seems to control how the two chipsets cooperate and, by default only the 2 Marvell controlled ports are available without it. There's no driver for Win11. Looks as though the phantom drive on my PC is Windows doing a best guess at what that second chipset is, since Device Manager reports the Marvell controller as the parent.
Some Windows 11 users say that all ports are available anyway; others report that the card doesn't work at all. I have a SATA controller in device manager, installed when I plugged the card in and working fine, with two HDDs plugged in. Since I have no need (now) for the other ports, I've disabled the unknown device under Disk drives, and the phantom disk has disapeared from Disk Management and Diskpart.
That'll more than do for me - until I need more drives.