Fusion drive only shows the SSD partition
I had the very same issue encountered with a iMac that has 1Tb Fusion drive.  22Gb was reported as SSD and then the rest of 1Tb of the drive.  After digging deeper into this problem, I noticed that the iMac Fusion drive was improperly configured and that I had to reformat it and create a Logical Volume, then configure the Fusion Drive to Apple Spec.

I found this article that helped me a bit, but I had to do a little improvising to get this procedure to work.

https://www.lifewire.com/setting-up-fusion-drive-mac-2260165