upgrade past high Sierra after replacing fusion HDD with SSD?

I have a 2014 Mac mini with a fusion drive. About 2yrs back I replaced the HDD portion of the fusion drive with a SSD. I was and am still run High Sierra and a bootcamp partition with Windows 10. Both work fine and the Mac mini is nice and speedy.

I now want to upgrade OS X to Catalina but the Apple upgrade tells me my core storage is not supported. Anyone know why? As mentioned, this config is running fine with High Sierra and a bootcamp windows 10 partition.

Apple support has been no help thus far. Is there a way to upgrade to Catalina with my current fusion config (PCIe & SSD making up my fusion drive)? I don’t plan to disassemble and put back an HDD…. yet and only if no choice…. Hopefully I don’t have to fall back :-(