I have heard of this happening in the past but it has been a long time and I have no idea how or why it happened but it would appear the SSD and HD components in your fusion drive have become disconnected. I found this article on creating a fusion drive from back in 2014. I have performed the procedure in both directions and it is not difficult — copying and pasting the commands goes a long way toward avoiding any errors. Hopefully the same commands are still in MacOS. (I would check but my MacBook Pro succumbed to the swollen battery syndrome and is in the shop so I am working on my iPad.)

If we knew what it was we were doing, it wouldn't be called research, would it?

— Albert Einstein