I've just gone back to test it again and have discovered there may be a time element involved.
After my earlier test I accidentally forgot to unmount /tmp/dir2
I've just unmounted and re-mounted it and there was file corruption.
I've just repeated the test using 2 new directories, /tmp/dir3 and /tmp/dir4. Unmounting and remounting immediately shows that they are decrypting fine.
I'll keep an eye on the new directories and see what happens over time.
I've just gone back to test it again and have discovered there may be a time element involved.
After my earlier test I accidentally forgot to unmount /tmp/dir2
I've just unmounted and re-mounted it and there was file corruption.
I've just repeated the test using 2 new directories, /tmp/dir3 and /tmp/dir4. Unmounting and remounting immediately shows that they are decrypting fine.
I'll keep an eye on the new directories and see what happens over time.