I tried repeating your experiment, sans NFS and NTFS, and while I did note that the modification timestamp on the file did not change, the contents of the file did (i.e. the filesystem changed).
FWIW this is Debian bug #459703 (Google told me that). What I'm going to guess is happening in your instance is the NFS server is caching parts of the file, and it checks the timestamp to see if the cache is dirty. Since it doesn't see that file file is modified it assumes its cache isn't dirty and continues to use it. You could possibly get away with it by simply # touch individual.files.img Maybe, maybe not. Also, usually you don't need to use losetup unless you are using encryption or offsets or something else exotic. Usually # mount -o loop my.img /mnt/pt Will do the trick.
