Imagine this
/mnt/user/share1/file1.txt (this is a cache-only share)
From the command prompt, move the file to another share that is set to not use the cache.
mv /mnt/user/share1/file.txt /mnt/user/share2/file.txt
The file will remain on the cache drive in the share2 folder, in apparent violation of the use cache setting. IMO, not a big deal as you're basically bypassing the system, but this can happen on occasion depending upon how you've set up docker containers (in particular if for simplicity sake you simply map /mnt to /mnt and nothing else).
The "problem" here is that because share2 is set to not use the cache drive, mover will not move the file off of the cache drive to the array. Unfortunately though @jonp did come up with a neat little use case to leverage this behaviour of mover here which would preclude changing mover to fix this (although, I personally think that mover should always move files to fix this problem, and @jonp be damned )