I have been doing some more research and
THINK your issue is arising at least in part because of a change that has occurred in hoe MacOS handles Fusion Drives. Originally Fusion Drives were created using Apple's
core storage technology. But Apple has since dripped
core storage and in current versions of MacOS they are supporting Fusion Drives through
APFS. I have no clue why the Fusion Drive came apart in the first place but it occurs to me your current issue
MAY be the result of some fragments of core storage format remaining on the drive.
IF that is the case your problem
MIGHT be solved by
intentionally separating the Fusion Drive to remove any remaining traces of Core Storage formatting.
This Lifewire article is the most current I have found on how to do that. Hopefully after that you will be able to either put your Fusion drive components back together using APFS tools or, as I already previously suggested, leave them as separate drives with the SSD a dedicated System Drive and the HD as your Data drive. 🤞🤞🤞
