It took an extra year to make APFS compatible with fusion drives and it occurs to me First Aid might not have caught up completely.
I think you might be right. The engineers to whom Apple Support referred my First Aid issue say it has been reported quite a few times. Their primary concern was "why was I running First Aid?" The answer was I thought it reasonable to do so prior to performing a clone backup if I could not confidently continue to use Disk Warrior, which still seems to have some issues with APFS, to perform a preparatory routine disk check. Their question also seems reasonable but it looks like it's directed at establishing some other root
cause for the FA failure report rather than acknowledging that FA itself may not be up to scratch yet!
As I would rather not clone a disk that may have invalid Fusion data structures, and this drive is otherwise operating normally, I'm going to put my head in the sand and rely on more laborious alternative essential file backup. At least until 10.14.3 arrives and see, if anything, what happens to Disk First Aid.
