creating APFS fusion drive / conversion to APFS from HFS+ fusion drive fails - "you are trying to create an upside down Fusion"
I finally took the plunge and upgraded my iMac to Catalina.  It originally had the fusion drive with SSD and spinning hard drive.  However, I upgraded the HDD to a Samsung SSD and combined it with the original SSD to create one large CoreStorage logical volume (can’t call it “Fusion” anymore.”)

While trying to convert this logical volume, I ran into all of the same issues you did, so I was able to replicate the problem.  However, after about an hour of throwing to force things, I found the trick: reverse the order of the drives

What you think it should be:

$ diskutil APFS create disk0 disk1 “MyVolume”


Reverse the disk identifiers to make it work

$ diskutil APFS create disk1 disk0 “MyVolume”


(I’ll post all of the command output shortly...I’m posting from my iPad as Catalina installs onto a APFS “Fusion” drive).