When you ran Disk Utility First Aid did you select the volume, the container, or the drive?
I can't remember but I've just repeated First Aid at all three levels with the same failure result. The details are as follows
Running First Aid on “Container disk2”
NOTE: First Aid will temporarily lock the startup volume.
Verifying storage system
Using live mode.
Performing fsck_apfs -n -x -l /dev/disk1s2
Checking the container superblock.
Checking the fusion superblock.
Checking the EFI jumpstart record.
Checking the space manager.
Checking the space manager free queue trees.
Checking the object map.Checking the Fusion data structures.
error: fusion_mid_tree: btn: invalid o_oid (0x2ec2b)
Fusion data structures are invalid.
The volume /dev/disk1s2 could not be verified completely.
Storage system check exit code is 8.
Operation successful?? Blimey I can't imagine why with the error indicated! Googling shows that this and similar fusion drive errors are common but none of the discussions about them reveal a solution that gives me any confidence. I think I might get into a chat with Apple Support...