Oh, bless you (and bless Google for leading me here)! I was pulling my hair out all morning trying to get one of my docker containers to work... turns out it was trying to write to a share affected by this issue. I, also, have a "standard" array, but a ZFS cache. I was able to write directly to the disk (i.e. /mnt/disk4/data) but not through the "user" FUSE path (i.e. /mnt/user/data).